feat(self): build a searchable project skill

[oritwoen]

skilld self turns current workspace into a searchable project skill. It indexes source and docs, installs SKILL.md for selected agent, and makes skilld search "..." -p self return links back to real project files.

This is the useful one-shot part of #33. Watch mode can stay separate instead of blocking whole feature on file watcher.

[harlan-github-agent]

🤖 BLOCKED

Harlan Agent Kit posted this automated review. It is not Harlan's personal review or approval. AI open source policy. Human merge decision still required.

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• Open: Self-search links add project/ twice. Results resolve to .skilld/project/project/src/..., not the workspace file.. Next: Store source paths without project/, or remove it from formatting.

• Open: A checked-out .skilld symlink can redirect index replacement outside the project, including database deletion.. Next: Reject symlinks in .skilld/self before any database writes.