craft-cli: agent-first controls

craft-cli got a small agent-focused upgrade.

The main change is craft source: auto reads from Craft Desktop’s local database when available and falls back to the API, api skips local reads for Linux or headless machines, and local fails clearly if the local store is unavailable.

There are also a few new agent ergonomics:

• craft doctor --json for auth, API, source, and local-store health.
• craft agent-context for a stable machine-readable command manifest.
• craft which <capability> to map intent to the right command.
• --select to trim JSON output to the fields an agent actually needs.
• Broader --dry-run previews across write commands.

Nothing changes about the core architecture: local-first reads when possible, API writes, mutation journal for diff, log, and undo.