feat: add recent cwd path selection

[kerwin2046]

Problem: Users who frequently switch between working directories had to re-enter custom paths or scroll through all session-derived projects to find their desired directory. There was no persistence of manually entered or recently used paths — they were lost after navigating away.
Changes:

1. New API endpoint app/api/cwd/recent/route.ts

• GET /api/cwd/recent — returns the list of recent working directories from ~/.pi/agent/recent-cwds.json
• POST /api/cwd/recent — accepts { cwd: string }, prepends it to the recent list (deduplicated), caps at 10 entries, and persists to the JSON file

2. components/SessionSidebar.tsx

• On mount, fetches the recent paths list and stores it in local state
• Whenever the user selects a CWD (via project list, custom path, default directory, or worktree switch), a debounced effect records it as a recent path via POST /api/cwd/recent
• The CWD dropdown now includes a "Recent" section between the project list and the "Default directory"/"Custom path" actions
• Recent paths that already appear in the current project list are filtered out to avoid duplication
• Up to 5 recent paths are shown, each with a folder icon and left-ellipsized path label
• The "Recent" section is hidden when the project filter is active to avoid confusion

[kerwin2046]

[agegr]

Thanks for the contribution. The current CWD selector already lists session-backed projects ordered by recent activity, with filtering for larger lists.

We’d like to keep the selector scoped to directories that have sessions, so we don’t need a separate persisted “Recent paths” list at this time. We’ll close this PR for now. Thanks for understanding.