chore: add CLAUDE.md with stack conventions and control-plane pointer

CLAUDE.md

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+# picoclaw — Claude Code Rules
+
+## What This Is
+Tiny, fast, deployable-anywhere automation CLI — part of the claw family of AI assistants.
+
+## Stack & Conventions
+- **Go** (module `github.com/sipeed/picoclaw`). CLI entry in `cmd/`, config in `config/`, docker in `docker/`.
+- Tests: `go test ./...`. Build: `make build` (see `Makefile`).
+- This repo is a nuestra-ai fork that tracks an upstream; be conservative about restructuring that would conflict with upstream merges.
+
+## Control Plane
+Directives, skills, agent definitions, and the broader context tree live in the **agent-platform umbrella repo** at `../agent-platform/`. For cross-repo work or any `/directive` pipeline usage, open Claude from there:
+
+```bash
+cd ../agent-platform && claude
+```
+
+## Git Operations
+NEVER perform git operations without explicit user approval.