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/activity/workspaceActivityService.test.ts

@@ -32,6 +32,19 @@ describe("WorkspaceActivityService", () => {
     expect(events.at(-1)).toMatchObject({ type: "workspace.activity", activity: { cwd: "/repo", hasSessionActivity: false, hasTerminalActivity: false } });
   });
 
+  it("does not report a workspace active for a session that is only starting up", () => {
+    const events: RealtimeEvent[] = [];
+    const service = new WorkspaceActivityService({ publishRealtime: (event) => events.push(event) });
+
+    // Startup progress names a phase the daemon is inside; it is not work, so the
+    // workspace (and the project indicators and remote machines that read it)
+    // must not be reported as busy because of it.
+    service.applySessionActivity("/repo", { sessionId: "s1", phase: "active", label: "Opening session", detail: "Starting the Pi session", at: "now", startup: true });
+
+    expect(service.snapshot().workspaces).toEqual([]);
+    expect(events.at(-1)).toMatchObject({ type: "workspace.activity", activity: { cwd: "/repo", hasSessionActivity: false } });
+  });
+
   it("clears stale active activity when an idle status arrives", () => {
     const events: RealtimeEvent[] = [];
     const service = new WorkspaceActivityService({ publishRealtime: (event) => events.push(event) });