fix(sessions): make the discard a real inverse of the construction

Third review of the reboot-restore branch. The narrow discard the previous
commit introduced avoided everything cleanupSession() did wrongly, and in
dropping so much of it also dropped four things it had to keep.

The worst broke the retry the whole design rests on. setupSessionListeners()
returns early while sessionListenerRefs still holds the session id, and the
discard never cleared that entry. So the advertised flow — a rebuild fails
because the agent binary is missing, the user fixes their PATH and clicks
again — reused the same id, wired no listeners at all, and produced a tab
that never showed output, never updated its status and never persisted. That
is worse than the leak the discard was added to prevent. Three more
registrations leaked with it: a RunSummaryTracker and its interval, an image
watcher on the workspace, and the Ralph fix-plan watcher. The discard now
undoes each registration setupSessionListeners() makes, in its order, and
the per-session custom-model config directory, which holds the endpoint's
API key literally and which nothing else would ever remove.

The image-watcher flag was restored after the code that reads it, so a
session came back reporting the feature as on with nothing watching. It
moves to the before-spawn phase, and that phase now runs before the
listeners rather than after them.

The generation counter that lets a mid-restore dismiss win was global while
clear() is ownership-scoped, so one user's dismiss discarded another user's
unspent entries, permanently, because nothing rebuilds an in-memory plan. It
is now per owner. Bumping only the owners of entries the dismiss removed was
not enough either: take() has already emptied the plan by then, so a dismiss
landing mid-restore saw nothing of that owner's to remove and invalidated
nothing. The owners that matter are those with a restore in flight, filtered
by what the dismissing user may access, and that is what clear() now bumps.
Plan expiry bumps too, so a restore straddling the 24-hour boundary cannot
hand entries back and give an expired plan another full day.

Tests. discardPartiallyBuiltSession had no test at all: the only
implementation any test ran was the mock's one-line stub, which is why every
defect above was invisible. test/discard-partially-built-session.ts drives
the real WebServer, and the retry assertion fails if the listener refs are
left behind — verified by reverting the fix. The dismiss-race test drove the
registry by hand, so deleting the route's generation argument left it green;
it now goes through the route, and two further tests cover the multi-user
cases.

The mock context has now gone stale twice, because route tests pass it as
`ctx as never` and tsconfig.json includes only src, so nothing ever compares
it to the ports. A type-level guard is therefore inert — I wrote one and
confirmed it never fires. test/mocks/mock-route-context-completeness.ts
compares the mock's keys against WebServer.createRouteContext() at runtime
instead, and names what is missing.

Also: the API reference now says workspace-forbidden is judged against the
owner's grant, the banner's module header no longer claims Restore always
dismisses it, and the detail span gets the same min-width: 0 the phone rule
already needed.

Refs #411

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

test/routes/reboot-restore-rebuild-failure.test.ts

@@ -282,18 +282,62 @@ describe('a failure before any entry is considered', () => {
 });
 
 describe('a dismiss that lands while a restore is running', () => {
-  it('wins, rather than being undone when the restore hands its entries back', async () => {
-    const entries = [offerEntry('a')];
-    rebootRestoreRegistry.set(entries);
-    const generation = rebootRestoreRegistry.currentGeneration();
-    const taken = rebootRestoreRegistry.take(() => true);
-    expect(taken).toHaveLength(1);
+  it('wins, rather than being undone when the route hands its entries back', async () => {
+    rebootRestoreRegistry.set([offerEntry('a')]);
+    const ctx = createMockRouteContext({ workspaceHooksEnabled: false });
+    // The user clicks Dismiss while the restore is between its take and its
+    // return. Driven through the ROUTE, so removing the generation argument from
+    // the route would make this fail.
+    (ctx.getWorkspaceHooksEnabled as ReturnType<typeof vi.fn>).mockImplementation(async () => {
+      rebootRestoreRegistry.clear(() => true);
+      throw new Error('settings unreadable');
+    });
+    const app = await createHarness(ctx);
 
-    // The user clears the banner while the restore is still working.
-    rebootRestoreRegistry.clear(() => true);
-    // The restore finishes and tries to put its unspent entry back.
-    rebootRestoreRegistry.restore(taken, generation);
+    await app.inject({ method: 'POST', url: '/api/reboot-restore/restore', payload: {} });
+
+    const left = (await app.inject({ method: 'GET', url: '/api/reboot-restore' })).json().data;
+    expect(left.sessions).toEqual([]);
+    await app.close();
+  });
+
+  it('reaches an in-flight restore the dismisser can see, even once its entries are taken', async () => {
+    const mine = offerEntry('mine', 'alice');
+    rebootRestoreRegistry.set([mine]);
+    expect(rebootRestoreRegistry.beginSpending('alice')).toBe(true);
+    const generations = rebootRestoreRegistry.snapshotGenerations([mine]);
+    const taken = rebootRestoreRegistry.take((owner) => owner === 'alice');
+
+    // The plan is empty now, so a dismiss has nothing of Alice's to remove; the
+    // invalidation has to come from her claimed flight.
+    rebootRestoreRegistry.clear((owner) => owner === 'alice');
+    rebootRestoreRegistry.restore(taken, generations);
+    rebootRestoreRegistry.endSpending('alice');
 
     expect(rebootRestoreRegistry.list(() => true)).toEqual([]);
   });
+
+  it('does not reach another owner, whose unspent entries still come back', async () => {
+    const mine = offerEntry('mine', 'alice');
+    const theirs = offerEntry('theirs', 'bob');
+    rebootRestoreRegistry.set([mine, theirs]);
+
+    // Bob is mid-restore, holding his own entry. The claimed flight is what makes
+    // this the interesting case: a dismiss can no longer see Bob's entries in the
+    // plan, so the invalidation has to come from the in-flight set, filtered by
+    // what the dismissing user may access.
+    expect(rebootRestoreRegistry.beginSpending('bob')).toBe(true);
+    const bobsGenerations = rebootRestoreRegistry.snapshotGenerations([theirs]);
+    const bobsTaken = rebootRestoreRegistry.take((owner) => owner === 'bob');
+    expect(bobsTaken.map((e) => e.sessionId)).toEqual(['theirs']);
+
+    // Alice dismisses her own banner meanwhile.
+    rebootRestoreRegistry.clear((owner) => owner === 'alice');
+
+    // Bob's restore finishes and hands his entry back. Alice's dismiss covered
+    // her entries, not his, so his offer survives.
+    rebootRestoreRegistry.restore(bobsTaken, bobsGenerations);
+    rebootRestoreRegistry.endSpending('bob');
+    expect(rebootRestoreRegistry.list(() => true).map((e) => e.sessionId)).toEqual(['theirs']);
+  });
 });