Add worktree chat completion feedback

[jln13x]

Summary

• open a new draft in the exact worktree when its sidebar row is activated
• send native macOS completion notifications that deep-link to the completed thread
• increase the completion sound gain by 10%
• emphasize active, selected, and newly completed thread titles
• add a default-on macOS notification feature flag and update the personal fork inventory

Verification

• vp check
• vp run typecheck
• vp test run: 4,828 passed, 7 skipped
• focused coverage: 113 passed