Announced pages: event windows, header announcements, summary tasks

A question may carry event: {starts, duration, place}. While the
window is open the page is announced in a strip at the top of every
header (name, when, 'in 3 days'), soonest first, and kept out of the
footer nav; when it closes the page becomes a followup - only a
visitor carrying an answer chain still sees it.

announce.rs keeps one record per event page in the runtime-owned
portal_events bucket (built-in state graph: announced ->
awaiting_summary -> summarized, content may override) and, on a
one-minute idempotent sweep, moves ended windows to awaiting_summary,
publishing the transition on portal.answers.submitted as 'Summary
due' - the post-what-happened task a review desk picks up. Lint
warns when event pages exist but nothing reads portal_events.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -39,6 +39,13 @@ stream.
   `requires_chain` gates a page on verifiable answer provenance next
   to `qualifies`' Kanidm-group identity gate (see
   `docs/design/filesystem-routes.md`).
+- **Announced pages** (`src/announce.rs`): a question with an
+  `event: {starts, duration, place}` window is announced in the
+  header (not the footer nav) while the window is open, becomes a
+  followup when it closes, and its record in the runtime-owned
+  `portal_events` bucket moves to `awaiting_summary` - a "post what
+  happened" task any review desk can read, published on NATS like a
+  decision. A one-minute sweeper keeps it all idempotent.
 - **State machines as content** (`src/aggregates/`): `aggregates.yaml`
   declares each bucket's states and legal transitions; the engine
   replays a record's event history and refuses undeclared moves, with