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

docs/guides/user-guide.md

@@ -309,6 +309,39 @@ mosaic quality-rails
 
 ---
 
+### Claude Code Skill Registration
+
+Mosaic stores canonical skills under `~/.config/mosaic/skills/`. Claude Code scans
+`~/.claude/skills/`, so Mosaic maintains one symlink per skill between those
+directories.
+
+```bash
+mosaic skill list
+mosaic skill register <name>
+mosaic skill unregister <name>
+```
+
+- `register` is idempotent and repairs a dangling Mosaic-owned link. Names use
+  the safe grammar `[A-Za-z0-9][A-Za-z0-9._-]*`; files, directories, foreign
+  symlinks, path traversal, absolute paths, and names beginning with `-` are
+  refused.
+- `unregister` is idempotent when no entry exists. It removes only symlinks that
+  point inside `~/.config/mosaic/skills/`; foreign entries are never removed.
+- `list` reports `registered`, `unregistered`, `dangling`, `foreign`,
+  `foreign-dangling`, or `misdirected` for each canonical or Claude entry.
+
+Install, wizard finalization, and `mosaic update` framework re-seeding reconcile
+every canonical skill automatically. A skill directory added after initial
+setup therefore receives its Claude bridge without a per-skill code change or
+manual `ln -s`. If Claude Code is already running, use `/reload-skills` or start
+a new session after registration so its in-process skill registry rescans.
+
+This command group is Claude-only in M1. Pi can consume Mosaic's canonical skill
+root through its Mosaic launcher configuration and does not need this Claude
+bridge. Codex has a separate link path managed by the legacy full skill-sync
+script; equivalent lifecycle management remains follow-up scope and is not
+changed here.
+
 ## Sub-package Commands
 
 Each Mosaic sub-package exposes its full API surface through the `mosaic` CLI.