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>

docs/api-reference.md

@@ -511,8 +511,9 @@ scrollbackRestored: false }`, ownership-scoped in multi-user mode.
 - `POST /api/v1/reboot-restore/restore` with `{ sessionIds?: string[] }` (omit
   to restore everything the caller can see) → `{ restored: RestorableSession[],
 skipped: { sessionId, reason }[] }`. `reason` is one of `workspace-missing`
-  (the directory is gone), `workspace-forbidden` (it is outside the caller's
-  workspace in multi-user mode), `already-live` (the conversation is already
+  (the directory is gone), `workspace-forbidden` (in multi-user mode it is
+  outside the workspace of the user the session belongs to, re-checked against
+  that owner's current grant rather than the caller's), `already-live` (the conversation is already
   open, typically resumed by hand from the Resume list), `capacity-reached`
   (the global or per-user session cap), or `rebuild-failed` (the agent would not
   start, most often a CLI binary missing from the server's PATH).