fix(opencode): track whether the session-URL notice is still pending before steering feedback (#1536)

#1518 made the OpenCode 2 client post the session-URL notice as a "steer" and,
while a notice had been posted, admit the reviewer's feedback as a "steer" too,
so the two share one promotion (#1515). The flag behind that, `noticePending`,
meant "we posted a notice during this review" rather than "our notice is still
an un-promoted inbox row", and nothing ever observed promotion.

The two diverge as soon as anything else wakes the session: a user typing an
unrelated message promotes every pending steer as one batch
(`SessionInbox.promote`), our notice included, and starts a turn. Feedback
arriving minutes later was still admitted as a steer, landing in the middle of
that unrelated turn — exactly the late-arrival case `FEEDBACK_DELIVERY =
"queue"` exists to prevent.

`createNoticePendingTracker` now answers the question from the host itself. It
subscribes to `ctx.event.subscribe()` before the notice is posted and clears the
flag when the host reports that row settled, speaking both live vocabularies:
`session.inbox.delivered` / `session.inbox.cancelled` with `data.inboxID`
(v2.0.x and dev) and the pre-rename `session.input.promoted` with
`data.inputID` (the `0.0.0-next-*` host this package pins). The row id comes
from `session.synthetic`'s own response. A host that reports no id, exposes no
event domain, or whose stream never delivers degrades to exactly the previous
behavior.

The client gains `dispose()`, called in a `finally` by both call sites, so a
review that ends without delivering a prompt does not leave an event
subscription open on the host.

A notice stranded by a review closed without feedback is unchanged: the plugin
session domain exposes no way to withdraw a pending row.