feat: park a session to set it aside

[jongracecox]

Summary

Adds Park alongside Stop as a way to end a session: same operation, different
intent. A parked session renders 🅿️ instead of 🟠 and sits out the bulk restore.

Why

Stopping is right for a break, but not for work you are consciously shelving. A stopped
session lands in the same orange bucket as everything a restart took away, and
Restore N stopped sessions sweeps it back up with the rest — so shelved work keeps
coming back.

Design

Parking is a flavour of stopped, not a third intent. SessionRecord.parked is a flag
on the existing open record, so is_stopped stays true for a parked session and
everything keyed off it — resume, terminate, placement in the sessions section —
keeps working untouched. Only two things change:

• the icon (🅿️ instead of 🟠)
• Restore N stopped sessions skips it, and its count excludes it

mark_open writes a fresh record, so launching or resuming un-parks — the natural exit,
mirroring how a stopped session leaves that state.

🟢 running        ← in flight
🅿️ parked         ← set aside on purpose
🟠 stopped        ← a restart or an out-of-band kill took it away
───── inactive worktrees ─────
⚫ terminated / never launched

Changes

• session_state.py — SessionRecord.parked, coerced with bool() on load so an older
  record still loads. touch(..., parked=None) is tri-state: None leaves the flag
  alone, so a plain stop of a parked session doesn't silently un-park it.
• cli.py — ICON_PARKED, SessionInfo.is_parked, _end_session(..., park=...),
  Park in the Ctrl-A x prompt (Terminate / Park / Stop / Cancel) and in the session
  actions menu, and the running → parked → stopped ordering in the home list.
• Docs — CLAUDE.md (session status table, module responsibilities, design decisions),
  README.md (icon legend, home screen, actions table) and CONTRIBUTING.md (manual test
  steps).

Testing

1002 tests pass. New coverage for the icon, the row ordering, the restore exclusion, the
flag round-trip (park → launch clears it), a plain stop leaving the flag alone, and both
entry points (menu item and prompt).

Compatibility

A record written before this change has no parked key and loads as unparked. An older
fujimoto reading a parked record ignores the field — it shows the session as 🟠 and will
include it in a restore, but nothing is corrupted and the flag survives.