(main): release the viewer-panel file watchers when the window closes

[jbr-sekoia]

The defect

The window-closed handler (main.js) releases the PTYs in activeSessions,
calls changesWatchers.closeAll() and drains subagentWatchers. It does not
touch fileWatchers, the registry behind watch-file / unwatch-file.

That registry is released only by an explicit unwatch-file, and a closing
window never sends one: on macOS the app survives the last window, so every
file a ViewerPanel had open keeps its fs.watch descriptor for the life of
the process, along with the debounce timer armed from inside it.

The fix

A closeAll-style helper next to the map, and one call in the closed
handler beside the two registries already drained there. The helper swallows
a close() that throws, which an fs.watch handle whose file is already gone
can do — matching git-changes-watch.js's disarm() rather than inventing a second
convention. A throw here would strand the subagent teardown that follows.

Deliberately out of scope

#301 also asks whether a single registry could serve all three callers
(fileWatchers, changesWatchers, subagentWatchers). That unification is
being folded into a larger piece and is not touched here: this PR changes
nothing about what any watcher does while armed, and nothing about
watch-file / unwatch-file behaviour. The issue stays open.

How it is pinned

test/window-close-releases-watchers.test.js:

• a source-text assertion that the closed handler names all three
  registries — the house pattern for main.js handlers
  (test/read-file-for-panel-bounds.test.js states it, and
  test/git-changes-watch.test.js already pins the sibling's closeAll the
  same way);
• the helper lifted out of the source and exercised: every watcher closed and
  the map emptied, and a watcher whose close() throws leaving the rest of
  the drain intact.

Mutation run: removing the closeAllFileWatchers() call from the closed
handler turns the file red at 1 fail / 4 tests; restoring it returns 4 pass.

Checks

• npx eslint . — 0 errors, 333 warnings (unchanged from c939748).
• npm test — batch 1 tests 1905 / pass 1902 / fail 1 / skipped 2, batch 2
  tests 120 / pass 119 / fail 0 / skipped 1. The single failure is the
  pre-existing environmental one, test/ipc-path-validator.test.js
  "allows files under ~/.claude/".

Refs #301.

[devsuitup]

Reviewed 9281fca. closeAllFileWatchers() sits beside the map, is called between changesWatchers.closeAll() and the subagentWatchers drain, and its per-watcher try/catch means a throwing close() cannot strand the steps after it; fileWatchers is module-level and emptied, so a macOS activate starts clean. The source-text assertions read the real main.js; the behavioural tests re-evaluate a regex-sliced copy of the helper — the house pattern, sound at this shape. CI green, no trailers.

One gap for parity with the sibling: the watch-file handler's debounce is a let closed over inside the handler (main.js:1010-1018) and nothing in closeAllFileWatchers can clear it. Harmless today — the callback checks mainWindow && !mainWindow.isDestroyed() and mainWindow is nulled synchronously in the same handler — but subagentWatchers' teardown explicitly clears its timers, and this "release" does not. Store the handle with the watcher and clearTimeout it in the drain; one test line.

Approving; the timer is a follow-up-sized change, before or after merge as you prefer.