fix(composer): show Cursor plugin skills

[smsunarto]

Human comments

What was wrong

Cursor skill discovery covered conventional skill directories, but not plugins installed under the local plugin directory or completed marketplace cache. The composer therefore omitted skills and commands supplied by installed Cursor plugins. This is a follow-up to #1292.

What changed

• Added a Cursor native-roots resolver for local and marketplace plugins.
• Parsed Cursor plugin manifests and forwarded declared skills and commands.
• Selected the newest completed marketplace revision and ignored incomplete cache entries.
• Deduplicated local links that resolve to the same cached plugin.
• No host-daemon protocol change.

How you verified

• pnpm exec turbo run test typecheck --filter=bb-plugin-provider-acp --force
• 81 provider ACP tests passed across 8 files.
• Provider ACP typecheck passed.

AGENT GENERATED