refactor(skills): remove ce-onboarding skill (#639)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

AGENTS.md

@@ -70,6 +70,9 @@ When changing `plugins/compound-engineering/` content:
 - Do not hand-bump release-owned versions in plugin or marketplace manifests.
 - Do not hand-add release entries to `CHANGELOG.md` or treat it as the canonical source for new releases.
 - Run `bun run release:validate` if agents, commands, skills, MCP servers, or release-owned descriptions/counts may have changed.
+- When removing a skill, agent, or command, add its name to both cleanup registries so stale flat-install artifacts are swept on upgrade:
+  - `STALE_SKILL_DIRS` / `STALE_AGENT_NAMES` / `STALE_PROMPT_FILES` in `src/utils/legacy-cleanup.ts`
+  - `EXTRA_LEGACY_ARTIFACTS_BY_PLUGIN["compound-engineering"]` in `src/data/plugin-legacy-artifacts.ts`
 
 Useful validation commands: