fix(sessions): correlate startup progress by token instead of workspace

Startup progress could still be shown on the wrong session's row. Routing by
known session id first closed the case where the browser knew the other
session, but left open the case where it does not -- which the browser is
designed to produce. While a create is pending for a workspace,
applyCreatedSession deliberately withholds a session.created event for that
workspace and stashes it, to avoid a duplicate row. So during exactly the
window this feature exists for, a session created by an agent's spawn or by
another tab is intentionally absent from the session list. Its startup events
carried an unrecognised id and a matching cwd, and were routed onto the user's
pending create row, showing a phase and a label belonging to another session.

Workspace path was never evidence of identity; it was the only key both sides
happened to share. Give them a real one. The browser already invents a
temporary row id for a pending create, so it now sends that id with the create
request as an opaque startupToken; the daemon carries it through construction,
echoes it on the startup events it publishes for that construction, and the
browser matches it exactly. The token is a throwaway label the daemon never
interprets. It never becomes the session id: activity.sessionId still carries
Pi's SessionManager id, which remains how an open of an already-known session
is routed.

With exact identity available, the guessing is deleted rather than gated.
startupProgressPendingStart goes entirely, and with it the selected-machine
comparison, the cwd filter, and the single-match ambiguity rule: a second
concurrent create carries a different token, and a foreign workspace or
non-selected machine carries no token this browser is waiting on, so those
cases stop existing rather than needing detection. One Map lookup replaces a
filtered scan. cwd comes off the event, since it existed only as the routing
key and nothing else read it.

No compatibility path is needed. session.startup is unreleased -- checked
against the published tarball, not only git tags -- so no deployed daemon
emits these events and no deployed browser parses them. An older daemon
ignores the extra request field; a newer daemon talking to an older browser
degrades to the pre-existing generic wording, as does any unmatched token.

One silent behaviour change to state plainly: startupProgress guarded on
`sessionId === "" || cwd === ""`. Removing cwd from the event removes the
meaningful half of that guard, and that half had no test. The session-id half
is kept, which is the half that actually protects honest reporting.

The replaced ambiguity test is rewritten rather than dropped, so the same three
scenarios still pin the user-visible guarantee -- no match means the generic
wording stays -- now including the reproduced foreign-session case, which fails
against the previous code. Session creation ordering, semantics, and queueing
are unchanged; the token is a passthrough label read only to build an event.

src/server/sessions/sessionRoutes.ts

@@ -45,10 +45,14 @@ export function registerSessionRoutes(app: FastifyInstance, sessions: SessionRou
     }
   });
 
-  app.post<{ Body: { cwd?: unknown } | undefined }>(`${prefix}/sessions`, async (request, reply) => {
+  app.post<{ Body: { cwd?: unknown; startupToken?: unknown } | undefined }>(`${prefix}/sessions`, async (request, reply) => {
     try {
       const body = requireRecord(request.body);
-      return await sessions.start(normalizeRequestCwd(requireString(body, "cwd")));
+      // An opaque label the caller uses to recognise its own construction's
+      // startup reports. Optional: only a browser row waiting for a session id
+      // has anything to correlate.
+      const startupToken = body["startupToken"] === undefined ? undefined : requireNonEmptyString(body, "startupToken");
+      return await sessions.start(normalizeRequestCwd(requireString(body, "cwd")), optionalField("startupToken", startupToken));
     } catch (error) {
       return reply.code(400).send({ error: errorMessage(error) });
     }