feat(penpal): live-detect worktree additions and removals

[loganj]

Summary

Previously, Penpal discovered git worktrees once at startup and never refreshed the list. If you ran git worktree add or git worktree remove while Penpal was running, the sidebar's worktree list went stale — showing deleted worktrees and missing new ones.

Now, worktree additions and removals are detected automatically via the existing workspace directory watch. When git worktree add ../foo creates a new sibling directory, the workspace watch fires a rescan that discovers the new worktree. Removal works the same way. No restart needed — the sidebar updates within moments.

Design: zero-cost detection

The implementation adds no additional inotify watches, no subprocess calls in the watcher hot path, and no lock acquisitions in the event loop. It piggybacks entirely on the workspace directory watch that was already in place:

1. git worktree add ../foo creates a new directory in the workspace
2. Workspace watch fires → debounced rediscoverProjects
3. Discovery runs git worktree list, updates cache, broadcasts SSE projects event
4. Frontend re-fetches /api/projects and the sidebar reflects the change

Changes

• discovery/worktree.go — gitCommonDirFS resolves the shared .git directory using pure filesystem reads (os.Lstat + os.ReadFile) instead of spawning git rev-parse. Used by discovery, not by the watcher.
• watcher/watcher.go — Extracted rediscoverProjects helper (serialized with rediscoverMu), tightened .md-only event filter, removed dead code. No worktree-specific watches or event handling.
• discovery/worktree.go — Removed dead code (FindMainWorktree, ResolveWorktree)
• PRODUCT.md / ERD.md / TESTING.md — Added E-PENPAL-WORKTREE-WATCH requirement and updated P-PENPAL-WORKTREE to specify live updates

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