Core: surface worktree failure details

[gabelul]

• Worktree creation failures now include the real git stderr/stdout in feature.last_error and emit an activity event (worktree.create_failed), instead of just "Worktree creation failed".
• WorktreeManager runs git worktree prune before add, and refuses to proceed if an old worktree directory couldn't be deleted (common on Windows file locks).
• Resolve Blockers modal now has an "Open example" button so you can jump straight to the feature details/logs for any blocker group.

How to verify:

• Trigger a worktree failure (e.g. lock the worktrees dir), then open the blocked feature: you should see the full git error.
• Open Resolve blockers → click "Open example" → feature modal shows last_error.

Tests:

• pytest -q
• npm -C ui run build