refactor(session): simplify code reuse and clean up interfaces

- Replace hand-rolled atomic file writes in saveIndexLocked() and
  writeSessionSnapshot() with fileutil.WriteFileAtomic(), gaining
  parent-dir fsync for flash storage durability (+12 existing callers)
- Extract duplicated filename validation into validateSessionFilename()
- Use utils.Truncate() in sessionLabel() instead of inline rune slicing
- Remove unused route parameter from buildCommandsRuntime()
- Remove ResolveActive from SessionOps interface (no handler uses it;
  only the agent loop calls it directly on agent.Sessions)

Net: -68 lines, 0 behavioral changes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/agent/loop.go

@@ -1554,7 +1554,7 @@ func (al *AgentLoop) handleCommand(
 		return "", false
 	}
 
-	rt := al.buildCommandsRuntime(route, agent)
+	rt := al.buildCommandsRuntime(agent)
 	executor := commands.NewExecutor(al.cmdRegistry, rt)
 
 	var commandReply string
@@ -1584,10 +1584,7 @@ func (al *AgentLoop) handleCommand(
 	}
 }
 
-func (al *AgentLoop) buildCommandsRuntime(
-	route routing.ResolvedRoute,
-	agent *AgentInstance,
-) *commands.Runtime {
+func (al *AgentLoop) buildCommandsRuntime(agent *AgentInstance) *commands.Runtime {
 	rt := &commands.Runtime{
 		Config:          al.cfg,
 		ListAgentIDs:    al.registry.ListAgentIDs,

pkg/commands/cmd_session.go

@@ -8,6 +8,7 @@ import (
 	"time"
 
 	"github.com/sipeed/picoclaw/pkg/session"
+	"github.com/sipeed/picoclaw/pkg/utils"
 )
 
 func sessionCommand() Definition {
@@ -109,11 +110,7 @@ func sessionLabel(summary, preview string, maxLen int) string {
 		return "(empty)"
 	}
 	text = strings.ReplaceAll(text, "\n", " ")
-	runes := []rune(text)
-	if len(runes) <= maxLen {
-		return text
-	}
-	return string(runes[:maxLen]) + "..."
+	return utils.Truncate(text, maxLen)
 }
 
 // extractSessionTag returns the "#N" suffix from a session key, or "#1" for

pkg/commands/runtime.go

@@ -9,7 +9,6 @@ import (
 // rely on. Implementations are expected to be scope-aware and deterministic
 // for a given scopeKey. SessionManager satisfies this interface.
 type SessionOps interface {
-	ResolveActive(scopeKey string) (string, error)
 	StartNew(scopeKey string) (string, error)
 	List(scopeKey string) ([]session.SessionMeta, error)
 	Resume(scopeKey string, index int) (string, error)

pkg/session/manager.go

@@ -12,6 +12,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/sipeed/picoclaw/pkg/fileutil"
 	"github.com/sipeed/picoclaw/pkg/providers"
 )
 
@@ -654,39 +655,7 @@ func (sm *SessionManager) saveIndexLocked() error {
 		return err
 	}
 
-	tmpFile, err := os.CreateTemp(sm.storage, "index-*.tmp")
-	if err != nil {
-		return err
-	}
-
-	tmpPath := tmpFile.Name()
-	cleanup := true
-	defer func() {
-		if cleanup {
-			_ = os.Remove(tmpPath)
-		}
-	}()
-
-	if _, err := tmpFile.Write(data); err != nil {
-		_ = tmpFile.Close()
-		return err
-	}
-	if err := tmpFile.Chmod(0o644); err != nil {
-		_ = tmpFile.Close()
-		return err
-	}
-	if err := tmpFile.Sync(); err != nil {
-		_ = tmpFile.Close()
-		return err
-	}
-	if err := tmpFile.Close(); err != nil {
-		return err
-	}
-	if err := os.Rename(tmpPath, sm.indexPath); err != nil {
-		return err
-	}
-	cleanup = false
-	return nil
+	return fileutil.WriteFileAtomic(sm.indexPath, data, 0o644)
 }
 
 func (sm *SessionManager) ensureScopeLocked(scopeKey string, now time.Time) (*scopeIndex, bool) {
@@ -855,13 +824,8 @@ func (sm *SessionManager) writeSessionSnapshot(snapshot Session) error {
 	}
 
 	filename := sanitizeFilename(snapshot.Key)
-
-	// filepath.IsLocal rejects empty names, "..", absolute paths, and
-	// OS-reserved device names (NUL, COM1 … on Windows).
-	// The extra checks reject "." and any directory separators so that
-	// the session file is always written directly inside sm.storage.
-	if filename == "." || !filepath.IsLocal(filename) || strings.ContainsAny(filename, `/\`) {
-		return os.ErrInvalid
+	if err := validateSessionFilename(filename); err != nil {
+		return err
 	}
 
 	data, err := json.MarshalIndent(snapshot, "", "  ")
@@ -870,39 +834,14 @@ func (sm *SessionManager) writeSessionSnapshot(snapshot Session) error {
 	}
 
 	sessionPath := filepath.Join(sm.storage, filename+".json")
-	tmpFile, err := os.CreateTemp(sm.storage, "session-*.tmp")
-	if err != nil {
-		return err
+	return fileutil.WriteFileAtomic(sessionPath, data, 0o644)
 }
 
-	tmpPath := tmpFile.Name()
-	cleanup := true
-	defer func() {
-		if cleanup {
-			_ = os.Remove(tmpPath)
+// validateSessionFilename rejects filenames that would escape sm.storage.
+func validateSessionFilename(filename string) error {
+	if filename == "." || !filepath.IsLocal(filename) || strings.ContainsAny(filename, `/\`) {
+		return os.ErrInvalid
 	}
-	}()
-
-	if _, err := tmpFile.Write(data); err != nil {
-		_ = tmpFile.Close()
-		return err
-	}
-	if err := tmpFile.Chmod(0o644); err != nil {
-		_ = tmpFile.Close()
-		return err
-	}
-	if err := tmpFile.Sync(); err != nil {
-		_ = tmpFile.Close()
-		return err
-	}
-	if err := tmpFile.Close(); err != nil {
-		return err
-	}
-
-	if err := os.Rename(tmpPath, sessionPath); err != nil {
-		return err
-	}
-	cleanup = false
 	return nil
 }
 
@@ -912,8 +851,8 @@ func (sm *SessionManager) deleteSessionFile(sessionKey string) error {
 	}
 
 	filename := sanitizeFilename(sessionKey)
-	if filename == "." || !filepath.IsLocal(filename) || strings.ContainsAny(filename, `/\`) {
-		return os.ErrInvalid
+	if err := validateSessionFilename(filename); err != nil {
+		return err
 	}
 
 	sessionPath := filepath.Join(sm.storage, filename+".json")