[codex] Fix session scan liveness and stale detection

[boxp]

Summary

• Fix startup scan resurrecting dead sessions: scan-recent-sessions! now checks agent liveness before writing active (:running etc.) sessions that lack a pane-id — sessions whose agent process is :dead are skipped, :alive/:unknown are written (consistent with stale cleanup's conservative handling of :unknown)
• Skip expired terminal sessions (older than the closed TTL) during scans so completed history doesn't flood the TUI
• Fix pane-id-less sessions never going stale: stale-session? now falls back to a cwd-matched process-tree check instead of unconditionally treating them as alive when their cwd matches any live pane
• Make session-has-live-agent? public so the scan path reuses the same 3-state (:alive/:dead/:unknown) liveness logic

Ticket

Obsidian BOXP-69 (follow-up to #73)

Test plan

• make ci (262 tests, 0 failures)
• New tests: dead/alive/unknown liveness during scan, expired-terminal skip, pane-id-less stale fallback (dead → stale, alive/unknown → not stale)

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