(sessions): probe descriptor liveness on both sides, verify the pid before a tmux attach

[devsuitup]

Audit 2026-09-11, findings 2, 9, 10 and 6 (.work-files/switchboard/audit-fable-2026-09-11.md, local note), plus the comment sweep owed on #259.

Why

• A local CLI killed without a clean exit leaves ~/.claude/sessions/<pid>.json behind; since (sidebar): show CLI status and age for local sessions too #254 its last status surfaced on the row and nothing ever re-checked the pid — busy · Nh until the app restarted.
• The remote inventory had no liveness at all: the host dot counted dead descriptors as live sessions.
• Since (remote): keep descriptors on a failed refresh, let a manual refresh force reconnection #255 descriptors survive a failed refresh, so a stale one could point at a recycled pid; the attach probe read that process's TMUX= socket and attached (and, when solo, reconfigured) whatever session had the old name there.
• The probe / restore-on-detach ssh calls had no ConnectTimeout.

What

• cli-session-state.js: getStatus() keeps the pid and re-probes liveness lazily, throttled per pid (5 s, injectable clock); a dead pid drops the entry.
• remote-transport.js: LIST_COMMAND prints an STX-prefixed ALIVE:0|1 marker after each descriptor from /proc/<pid> — same ssh round-trip; parseSessions drops dead ones and reports the count, an older host output without markers is kept as before.
• remote-attach.js: the probe appends whether /proc/<pid>/cmdline names a claude CLI; an explicit "no" refuses the attach before any set or spawnPty (pid N now belongs to a process that is not a claude CLI). procStart matching was not used: the CLI's value is a Windows-style start time, not comparable to Linux starttime without a format the repo has not measured. ConnectTimeout=5 on the non-interactive ssh calls.
• Comment sweep on the (activity): remote decay clears busy without claiming response-ready; PTY purge skips remote rows #259 files (one-line pointers).

Proof

Mutations on the shipped modules: disable the lazy re-probe → "dead between two calls" test red; drop the ALIVE filter → dead-descriptor test red (the shell test runs the real LIST_COMMAND under sh -c with a live and a dead pid); drop the cmdline check → refuse test red.

task check (two-stage runner from #261): 1155 pass + 119 pass, 0 fail, 8 pre-existing skips.

Not verified on the wire: the ALIVE marker and cmdline segment against the real host (tests only).