fix(tmux): gate the pane-exit read, and mute the dot on the rich rail too

Four changes the maintainer asked for on Ark0N/Codeman#446 before merging.

The pane-exit watcher stays always-on, but a tick now costs nothing when there
is nothing to observe. `hasObservablePaneSession()` skips the tmux exec while
every session on the manager is one of the shapes `Session.paneExitApplies`
already forces to UNKNOWN: a remote SSH session (its local pane holds the ssh
client), a docker case (a `docker exec` into the container's own tmux), and a
record rebuilt from the socket (no provenance at all). The timer is untouched.
Skipping retracts nothing, for the same reason a failed read does not: the map
still holds the last real reading, and every path that puts a new command in a
pane calls `clearPaneExit()` itself. The two copies of that rule are pinned
against each other in `test/session-pane-exit.test.ts`, because drift between
them is silent in both directions.

`DEFAULT_PANE_EXIT_INTERVAL_MS` was already a constant beside the stats and
remote-reconnect intervals; its comment now says why the watcher owns its own
cadence and why the number is what it is.

The never-default-an-absent-status rule is written where `PaneExit` is declared.
It names `status ?? 0` as the thing never to write, and says that an agent the
OOM killer took would otherwise read as a user typing `/exit` — which is what
absent-stays-absent keeps a later clean-exit sweep away from. Nothing fails when
somebody adds that `??`, which is why the sentence is there rather than a test.

Checking the dot's specificity found a second fight, and it was losing. On the
tab strip the alert rules win as intended: a session that exits with a
permission dialog pending still renders red, and yellow for an idle alert. On
the rich vertical tab rail they did not — that rail's own `tab-state-*` dot
rules are (0,9,1) against the strip's mute at (0,5,0), so an exited session
there kept a full green dot AND the working halo beside a badge reading
"exited". The rail twin matches that specificity exactly and therefore must stay
below those rules in source order; it clears the halo as well, which the strip's
rule never had to think about.

`test/session-pane-exit-ui.test.ts` now resolves the real stylesheet in jsdom
rather than matching selector text: postcss collects every rule that paints
`.tab-status`, a real engine decides, and the tests read back the answer. Two
mutations were run against it to prove it has teeth — dropping the hand-written
alert exclusions fails three cases, and moving the rail twin above the state
rules fails one.

Refs Ark0N/Codeman#446.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/types/session.ts

@@ -645,9 +645,23 @@ export interface CustomModelBookkeeping extends CustomModelSelection {
  * reports `pane_dead=1` with BOTH `#{pane_dead_status}` and `#{pane_dead_signal}`
  * empty, and `#{pane_dead_signal}` does not exist at all before tmux 3.4. So an
  * absent `status` means "the exit code is unknown", never "the exit code is 0".
+ *
+ * ⚠ AN ABSENT `status` STAYS ABSENT. Never write `status ?? 0`, and never read
+ * "no signal was reported" as "the exit must have been clean". On tmux 3.2a
+ * the absent status IS how a signal death presents, so absent-stays-absent is
+ * the only thing keeping a future clean-exit sweep away from crashed agents:
+ * an agent SIGKILLed by the OOM killer would otherwise read as a user typing
+ * `/exit` and be swept. Nothing here fails when somebody adds that `??` — the
+ * types allow it, the label still renders, and the damage shows up only once
+ * the sweep lands. The rule is enforced in `derivePaneExits()`
+ * (`tmux-manager.ts`), which omits the key rather than defaulting it.
  */
 export interface PaneExit {
-  /** tmux `#{pane_dead_status}` — the command's exit code. Absent when tmux reported none. */
+  /**
+   * tmux `#{pane_dead_status}` — the command's exit code. Absent when tmux
+   * reported none, which means UNKNOWN and never 0. See the ⚠ above before
+   * giving this a default anywhere.
+   */
   status?: number;
   /** tmux `#{pane_dead_signal}` — the signal that killed the command. Absent when unsignalled or unsupported. */
   signal?: number;