feat(mosaic): add secure skill registration CLI (#826)

docs/guides/dev-guide.md

@@ -8,8 +8,9 @@
 4. [Adding New Agent Tools](#adding-new-agent-tools)
 5. [Adding New MCP Tools](#adding-new-mcp-tools)
 6. [Database Schema and Migrations](#database-schema-and-migrations)
-7. [API Endpoint Reference](#api-endpoint-reference)
-8. [Local Fleet Canary](./fleet-local-canary.md)
+7. [Claude Code Skill Bridge](#claude-code-skill-bridge)
+8. [API Endpoint Reference](#api-endpoint-reference)
+9. [Local Fleet Canary](./fleet-local-canary.md)
 
 ---
 
@@ -353,6 +354,37 @@ defined there.
 
 ---
 
+## Claude Code Skill Bridge
+
+The framework's canonical skill root is `~/.config/mosaic/skills/`; Claude Code
+requires registrations under `~/.claude/skills/`. The implementation in
+`packages/mosaic/src/commands/skill.ts` owns only direct-child symlinks whose
+resolved target remains inside the canonical root.
+
+Security invariants:
+
+1. Validate the user-supplied name before filesystem access against
+   `[A-Za-z0-9][A-Za-z0-9._-]*`. Separators, control characters, whitespace,
+   `..`, absolute paths, and leading `-` are invalid; filesystem-derived invalid
+   names are escaped before terminal output.
+2. Never replace a real file, directory, foreign symlink, or live misdirected
+   symlink in the Claude skill directory.
+3. Repair a dangling link only when its lexical target is inside the canonical
+   Mosaic skills root.
+4. Unregister only a symlink pointing inside that root.
+5. Enumerate canonical directories at runtime; never hardcode framework skill
+   names.
+
+`finalizeStage` reconciles after wizard/framework synchronization, and
+`runFrameworkReseed` reconciles after the sync-only `mosaic update` path. A
+foreign conflict is reported but does not prevent unrelated canonical skills
+from registering. Filesystem tests use injected temporary roots in
+`skill.spec.ts`, `finalize-skills.spec.ts`, and `update-checker.reseed.spec.ts`.
+
+M1 intentionally manages Claude Code only. Pi's Mosaic launcher can discover the
+canonical root directly. Codex still relies on the existing full skill-sync
+linker and needs separate parity analysis before this lifecycle API is extended.
+
 ## API Endpoint Reference
 
 All endpoints are served by the gateway at `http://localhost:14242` by default.