feat(push): plumb hostname-aware prefix into Web Push notifications

Closes the Web Push gap left by #82: in-page Notification API and tab
title flash both showed `codeman:<host>` after that PR, but OS-level
notifications dispatched via the service worker — the surface that
matters most when the tab is closed and the user is reading their
system notification center across multiple Codeman instances —
still hardcoded the literal "Codeman" prefix.

Service workers run in an isolated context with no access to
document.title or any in-page state, so the hostname has to ride
along in the push payload itself.

Server (server.ts:sendPushNotifications): emit `hostTitle: this.windowTitle`
in the JSON payload alongside the existing `title` (event-specific text
like "Permission Required"). The two stay separate so the SW can compose
them — the server knows the host, the SW knows the OS context.

Service worker (sw.js): compose `${hostTitle}: ${title}` when both
present, mirroring the in-page Notification format from
notification-manager.js. Fall back to `title || hostTitle || 'Codeman'`
so older servers (which omit hostTitle) keep working — the field is
purely additive on the wire.

Tests (test/push-payload-host-title.test.ts): mock the `web-push` module
via vi.hoisted(), instantiate WebServer without binding a port, stub
the push store with one fake subscription, and verify the JSON payload
shipped to webpush.sendNotification carries the right hostTitle for
both --title-hostname overrides and the os.hostname() default. Also
mirrors the SW's title-composition logic in a small helper so any
future change to the format breaks the test instead of being caught
only by users running multiple Codeman instances.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/web/server.ts

@@ -1437,6 +1437,10 @@ export class WebServer extends EventEmitter {
 
     const payload = JSON.stringify({
       title: template.title,
+      // Hostname-aware prefix so OS-level notifications from multiple Codeman
+      // instances (laptop / dev box / NAS) are unambiguous in the system tray.
+      // Mirrors the in-page Notification format in notification-manager.js.
+      hostTitle: this.windowTitle,
       body,
       tag: `codeman-${event}-${sessionId}`,
       sessionId,