fix(sessions): stop counting session startup as work in progress

Startup progress rides the per-session activity channel with an "active"
phase, because a startup phase really is in progress. But isSessionActive()
treated any active activity as work, so a session that was merely *opening*
enabled "Stop Active Work", disabled "Reload from disk" with the misleading
"Stop current session activity before reloading" tooltip, showed the row's
active-work indicator, and — for any caller that hands a startup activity to
WorkspaceActivityService — reported the whole workspace as busy. Selecting an
archived, read-only session reported active work while it opened.

Starting is not working. publishStartupProgress now marks its reports with a
new optional SessionActivity.startup field, and isSessionActive() does not
count a marked activity. Every affected consumer — the session list, the core
actions, the app's activity-transition handling, and the server's workspace
aggregation — reads that one helper, so the correction lands in all of them at
once.

The marker is a new field rather than a new phase on purpose: six readers test
phase === "active" directly, including the pending row's "creating · " prefix,
the chat dock's active styling, and the daemon's own heartbeat re-publication.
It can only ever remove the activity-phase reason for being active, so
streaming, bash, compaction, and queued prompts still report as active through
the status even while a startup report is the latest activity. The chat dock
still shows the startup text; this changes what counts as work, not what is
shown.

The browser's own pending-create row keeps its previous appearance: it borrows
only the daemon's phase text and drops the marker, since that row stands for a
create the user is waiting on rather than a session the daemon is opening.

src/server/sessions/piSessionService.ts

@@ -3039,10 +3039,15 @@ export class PiSessionService implements SessionRouteService {
    * Unlike {@link publishActivity} this deliberately records nothing: no
    * `activities` entry, no workspace activity, no unread observation. There is
    * no session to own that state, and a failed creation would leave it stranded.
+   *
+   * Every report is marked `startup`, which is what keeps a session that is
+   * merely opening from counting as one doing work. This is the only publisher
+   * that sets the marker, and because it writes no `activities` entry no later
+   * heartbeat re-publication can carry it.
    */
   private publishStartupProgress(sessionId: string, startupToken: string | undefined, label: string, phase: "active" | "idle", detail: string | undefined): void {
     const at = new Date().toISOString();
-    const activity = detail === undefined ? { sessionId, phase, label, at } : { sessionId, phase, label, detail, at };
+    const activity = detail === undefined ? { sessionId, phase, label, at, startup: true } : { sessionId, phase, label, detail, at, startup: true };
     this.events.publishGlobal(startupToken === undefined ? { type: "session.startup", activity } : { type: "session.startup", startupToken, activity });
   }