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/web/public/styles.css

@@ -18497,6 +18497,26 @@ html[data-tab-orientation='vertical'][data-tab-rail-detail='rich']:not(.tab-rail
   border-width: 2px;
 }
 
+/* The exited-agent mute, again, for the rich rail (Ark0N/Codeman#446).
+   The strip's rule is (0,5,0) and the three state rules above are (0,9,1), so
+   on this rail an exited session kept a full green dot and the working halo
+   beside a badge reading "exited" — measured, the contradiction the mute
+   exists to remove. This twin matches their (0,9,1) exactly and therefore MUST
+   stay below them in source order; moving it above silently restores the green
+   dot. It also clears the halo, which is a box-shadow the strip's rule never
+   had to think about.
+   ⚠ The alert exclusions are repeated by hand for the same reason they are on
+   the rules above: a dot turning red or yellow because a session is blocked on
+   a human outranks "the agent exited". */
+html[data-tab-orientation='vertical'][data-tab-rail-detail='rich']:not(.tab-rail-compact)
+  .tab-rail
+  .session-tab.tab-agent-exited:not(.tab-alert-action):not(.tab-alert-idle)
+  .tab-status {
+  background: var(--text-muted);
+  opacity: 0.5;
+  box-shadow: none;
+}
+
 /* Card accents: the same three colours as every other session surface, and the
    same two blinks the home rail runs. `tab-alert-*` draws its own ::before ring
    on top of this for the sessions that are genuinely blocked on a human; these