feat(gc): expire keep-tagged exited sessions after a max age - #173

Problem

keep=true exempted a dead session from pty gc's sweep forever. Agents set the tag on the session they are debugging right now (pty run --tag keep=true) and nobody ever comes back to untag it, so the registry becomes append-only. On dev3: 911 sessions, 65 live, 740 EXITED-and-kept — and pty gc --dry-run could only offer to sweep 113. Disk is negligible; the cost is registry noise that every pty ls and every agent-facing listing pays.

Implements decision Q7 (keep=true on an exited session is honored for N days, default 7, then swept). Refs schickling/dotfiles#2412, schickling/dotfiles#2408.

Change

• pty gc --keep-max-age <dur> (default 7d): a keep-tagged dead session is exempt from step 3's sweep only while it has been dead for less than the window. Grammar is the existing parseDuration (Ns/Nm/Nh/Nd), plus bare 0 meaning "sweep every dead keep session now". A unit-less non-zero value is rejected (--keep-max-age 7 is ambiguous between seconds and days) rather than guessed at.
• Applies to exited/vanished sessions only. Running sessions were never sweep candidates (isGone gate) and still aren't, at any age or window — covered by a test.
• Age anchor: exitedAt, falling back to createdAt for a vanished session whose daemon never wrote an exit record — the same precedence pty list --older-than uses. A record with neither timestamp never expires, except under an explicit 0; retaining an unaged record is the recoverable failure.
• Exit-time retention (shouldReapAtExit) is untouched and still unconditional. Expiry happens only on a gc pass.
• GcResult gains keepExpired: string[], disjoint from removed, so "the keep tag expired" is reportable apart from ordinary stale-session cleanup. --dry-run previews it and counts it separately (N keep-expired sessions in the summary bar). New public exports: DEFAULT_KEEP_MAX_AGE_MS, isKeepExpired.
• CLI output: Removed (keep expired after 7d): <name> / Would remove (keep expired after 7d): <name>; the retained line now names the countdown — Kept (keep tag): <name> — swept once dead for 7d, or remove the keep tag to reap it now.
• Docs: pty gc --help + top-level usage, completions spec (src/completions.ts) with the three checked-in artifacts regenerated, README (lifecycle table, examples, sweep + auto-gc sections), docs/disk-layout.md keep-tag semantics, CHANGELOG under Unreleased.

Gates + evidence

• npm run typecheck — clean (re-run after rebase onto Make pty kill verify and finish the kill, read the process table in one place, and stop waiting to say a name is taken #170).
• npx vitest run tests/gc-keep-expiry.test.ts — 8/8 pass. Covers: young keep session retained; expired one swept and reported apart from an identical untagged session; custom window in both --keep-max-age 3h and --keep-max-age=1h spellings; 0 sweeping a just-exited session; createdAt anchoring for a record with no exit record; a running keep session surviving --keep-max-age 0 (verified still status=running afterwards); --dry-run/-n non-mutating in both default and zero windows; flag validation.
• npx vitest run (full suite, pre-rebase commit) — 1625 passed, 5 failed in 3 files, all environment-bound and none touching gc:
  • tests/completions.test.ts > completes evidence modes and leaf-specific flags in bash — 5s test timeout; the bash completion probe takes ~40s on this host. Proven red at the same base with my changes stashed and dist/ rebuilt: identical single failure, 41s.
  • tests/shells.test.ts > zsh > starts up and accepts commands — execvp(3) failed.: No such file or directory; zsh is not on PATH in this environment.
  • tests/screenshot.test.ts vim + nano cases — vim reports E1568: Terminal did not respond to DSR request for 'background' color against the headless harness; fails identically when those two files run in isolation.
• npx vitest run tests/gc*.test.ts tests/help.test.ts tests/completions.test.ts tests/disk-layout-docs.test.ts tests/list-purity.test.ts — green except the pre-existing bash-completion timeout above. The "matches every checked-in completion artifact" parity test passes, so the regenerated completions/pty.{bash,zsh,fish} are in sync.
• Post-rebase re-run of tests/gc-keep-expiry.test.ts tests/gc.test.ts tests/help.test.ts — green (plus the same pre-existing completions timeout).
• Manual smoke against a scratch PTY_ROOT: 2-day-old and 30-day-old exited keep sessions → the default pass keeps the young one and reports Removed (keep expired after 7d): old; --keep-max-age 0 sweeps both; dry runs left both files on disk; --keep-max-age 7 exits 1 with the duration error.

Intentionally left

• No pty gc run on dev3 and no dotfiles pin bump — separate step after merge, per the work brief.
• No per-session override tag (e.g. keep.max-age=<dur>): nothing asks for per-session retention yet, and the global flag plus pty rm cover both directions.
• GcResult's doc header still says "Five buckets" (already inaccurate before this change, at 9 buckets) — left alone to keep the diff to one logical change.