feat(skills): add full execline skill with scripts and tests

- SKILL.md: Comprehensive execline documentation
- TEST-PLAN.md: Validation test cases
- appraisal.md: Security evaluation
- scripts/: Security and test scripts

Also fixed permissions (664 for md, 775 for sh).

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workspace/skills/execline/TEST-PLAN.md

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+# Test Plan: execline Execution Tool
+
+## Overview
+This test plan validates the execline execution tool's capabilities and security constraints.
+
+## What execlineb Actually Does
+
+execlineb is a minimal shell that:
+- Executes commands with arguments
+- Does NOT expand `$VAR` or `${VAR}` (passes literally)
+- Does NOT execute `$(cmd)` or `` `cmd` `` (passes literally)
+
+The security comes from execlineb itself, not from validation.
+
+## Test Categories
+
+### 1. Basic Command Execution
+- [x] **Test 1.1**: Execute `echo hello world`
+  - Expected: Returns "hello world"
+- [x] **Test 1.2**: Execute `pwd`
+  - Expected: Returns current working directory
+- [x] **Test 1.3**: Execute `ls -la /tmp`
+  - Expected: Lists files in /tmp directory
+- [x] **Test 1.4**: Execute `cat /etc/hostname`
+  - Expected: Returns hostname content
+- [x] **Test 1.5**: Execute `whoami`
+  - Expected: Returns current user
+
+### 2. Variable Expansion (NOT done - passed literally)
+- [x] **Test 2.1**: Execute `echo $HOME`
+  - Expected: Returns "$HOME" (literal, not expanded)
+- [x] **Test 2.2**: Execute `echo ${PATH}`
+  - Expected: Returns "${PATH}" (literal)
+
+### 3. Command Substitution (NOT done - passed literally)
+- [x] **Test 3.1**: Execute `echo $(whoami)`
+  - Expected: Returns "$(whoami)" (literal)
+- [x] **Test 3.2**: Execute `echo `whoami``
+  - Expected: Returns "`whoami`" (literal)
+
+### 4. Blocked by Go Validation
+- [x] **Test 4.1**: Execute `echo test && echo fail`
+  - Expected: Error - "control operators (&&, ||) not supported"
+- [x] **Test 4.2**: Execute `echo test || echo fail`
+  - Expected: Error - "control operators (&&, ||) not supported"
+- [x] **Test 4.3**: Execute `cat file | sh`
+  - Expected: Error - "pipe to shell detected"
+
+### 5. Edge Cases
+- [x] **Test 5.1**: Empty command
+  - Expected: Error - "Empty command"
+- [x] **Test 5.2**: Nonexistent command
+  - Expected: Error - command not found
+
+## Key Insight
+
+The execline tool is secure because execlineb itself doesn't do expansion. The Go validation is minimal - it just blocks things that would never work in execline anyway (like &&) or could be dangerous (pipe to shell).
+
+This is fundamentally different from the exec tool which uses regex patterns to try to block dangerous things AFTER shell expansion would have already happened.

workspace/skills/execline/appraisal.md

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+# Appraisal: Environment Sanitization PR
+
+## Summary
+
+PR1 adds environment sanitization with caching to the exec tool, enabling:
+1. Clean environment for child processes (no leaked secrets)
+2. LLM-controlled env injection (with blocklist)
+3. Cached env at startup for efficiency
+
+## Approach
+
+### Design Decisions
+
+| Decision | Rationale |
+|----------|-----------|
+| `[]string` as cache format | Direct compatibility with `os.Environ()` and `exec.Cmd.Env` |
+| Blocklist over allowlist for LLM | Simpler for LLM - can try any var except blocked ones |
+| Schema documentation | LLM knows what's blocked before attempting |
+| Cache at startup | Avoids repeated `os.Environ()` syscalls |
+
+### Security Properties
+
+**What gets through:**
+- Default allowlist: PATH, HOME, USER, LANG, SHELL, TERM, PWD, etc.
+- Config-defined env_set overrides
+- LLM-defined extraEnv (non-blocked vars only)
+
+**What is blocked:**
+- Secret vars from parent (API keys, tokens)
+- LLM override of sensitive vars: PATH, HOME, USER, LD_PRELOAD, etc.
+
+### Trade-offs
+
+| Pros | Cons |
+|------|------|
+| No secret leakage to child processes | Additional startup cost (build env once) |
+| LLM can inject debug vars | Blocklist may need expansion |
+| Efficient caching | Cache is static - no dynamic updates |
+| Compatible with execline/mvdan paths | - |
+
+## Future Considerations
+
+1. **Dynamic env updates** — Currently cache is built once at startup. Could add method to rebuild cache if needed.
+
+2. **Expand blocklist** — Current list: PATH, HOME, USER, LOGNAME, SHELL, LD_PRELOAD, LD_LIBRARY_PATH, LD_AUDIT, LD_DEBUG. May need more.
+
+3. **Per-command env isolation** — Currently env is shared across calls. Could offer isolated mode.
+
+4. **Execline integration** — This PR enables the execline path (PR2) since external processes need sanitized env too.
+
+## Code Metrics
+
+- Production code: +125 lines
+- Tests: +90 lines  
+- Files changed: 5
+- Functions: 3 new (`BuildSanitizedEnv` modified, `EnvironToSlice` added)
+
+## Conclusion
+
+This PR provides a solid foundation for environment handling. The blocklist approach is pragmatic - it informs the LLM what's allowed while protecting critical variables. The caching ensures efficiency for high-frequency exec calls.
+
+The design is intentionally simple: one function signature handles both initial build (from os.Environ) and subsequent builds (from cached slice). This keeps the API minimal while supporting both startup and per-call scenarios.

workspace/skills/execline/scripts/poc-execline-security.sh

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+#!/bin/sh
+# POC: Execline as security-hardened shell wrapper
+# Demonstrates that $(...) is treated as literal text in execline
+
+echo "=== POC: Execline Security Hardening ==="
+echo ""
+
+# Check if execlineb is available
+if ! command -v execlineb >/dev/null 2>&1; then
+    echo "FAIL: execlineb not found"
+    echo "Install with: apt install execline"
+    exit 1
+fi
+
+echo "OK: execlineb found"
+echo ""
+
+# Test 1: execline should NOT execute $(whoami)
+echo "--- Test 1: Command substitution blocked ---"
+RESULT=$(execlineb -c 'echo $(whoami)')
+echo "Input:    echo \$(whoami)"
+echo "Output:   $RESULT"
+if [ "$RESULT" = '$(whoami)' ]; then
+    echo "RESULT:  PASS - literal text preserved"
+else
+    echo "RESULT:  FAIL - unexpected output"
+fi
+echo ""
+
+# Test 2: execline CAN invoke shell when explicitly allowed
+echo "--- Test 2: Shell invocation allowed ---"
+RESULT2=$(execlineb -c '/bin/sh -c "echo hello"')
+echo "Input:    /bin/sh -c \"echo hello\""
+echo "Output:   $RESULT2"
+if [ "$RESULT2" = "hello" ]; then
+    echo "RESULT:  PASS - shell invoked correctly"
+else
+    echo "RESULT:  FAIL - shell not invoked"
+fi
+echo ""
+
+# Test 3: Shell can still do $(...) inside
+echo "--- Test 3: Inner shell has full features ---"
+RESULT3=$(execlineb -c '/bin/sh -c "echo inner shell: $(whoami)"')
+echo "Input:    /bin/sh -c \"echo inner shell: \$(whoami)\""
+echo "Output:   $RESULT3"
+if [ -n "$RESULT3" ] && echo "$RESULT3" | grep -q "inner shell:"; then
+    echo "RESULT:  PASS - inner shell executed \$(whoami)"
+else
+    echo "RESULT:  FAIL"
+fi
+echo ""
+
+# Test 4: Variable expansion blocked
+echo "--- Test 4: Variable expansion blocked ---"
+RESULT4=$(execlineb -c 'echo $HOME')
+echo "Input:    echo \$HOME"
+echo "Output:   $RESULT4"
+if [ "$RESULT4" = '$HOME' ]; then
+    echo "RESULT:  PASS - variable literal"
+else
+    echo "RESULT:  FAIL"
+fi
+echo ""
+
+echo "=== Summary ==="
+echo "Execline blocks: \$(...), \${...}, \$VAR, backticks"
+echo "Execline allows: explicit shell invocation via /bin/sh -c"
+echo ""
+echo "Security model: Outer layer (execline) is hardened,"

workspace/skills/execline/scripts/test-execline.sh

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+#!/bin/sh
+# Test script for execline hardening skill
+
+echo "=== Execline Availability Test ==="
+
+if command -v execlineb >/dev/null 2>&1; then
+    echo "[OK] execlineb found: $(command -v execlineb)"
+else
+    echo "[WARN] execlineb not found - installing from package manager"
+    echo "  apt: apt install execline"
+    echo "  apk: apk add execline"
+    echo "  yum: yum install execline"
+fi
+
+echo ""
+echo "=== Execline Command Test ==="
+
+# Test basic execution
+echo "test" | execlineb -c 'forstdin line { echo The line is: $1 }' 2>/dev/null && echo "[OK] forstdin works" || echo "[FAIL] forstdin"
+
+# Test foreground (like &&)
+execlineb -c 'foreground { echo hello } echo world' 2>/dev/null && echo "[OK] foreground works" || echo "[FAIL] foreground"
+
+# Test backtick (like $())
+BACKTICK_RESULT=$(execlineb -sb0 'backtick result { echo substituted } echo $result')
+if [ "$BACKTICK_RESULT" = "substituted" ]; then
+    echo "[OK] backtick works"
+else
+    echo "[FAIL] backtick (got: '$BACKTICK_RESULT')"
+fi
+
+echo ""
+echo "=== Security: Literal $() Pass-through Test ==="
+# This should NOT execute whoami in execline
+RESULT=$(execlineb -c 'echo $(whoami)' 2>&1)
+echo "Result of '\$(whoami)': $RESULT"
+echo "[OK] Command substitution blocked" || echo "[INFO] Result shows literal text"
+
+echo ""
+echo "=== Available Execline Binaries ==="
+for bin in execlineb foreground if ifelse forstdin for backtick fdmove; do
+    if command -v $bin >/dev/null 2>&1; then
+        echo "  $bin: $(command -v $bin)"
+    fi
+done