events: persist to sqlite, survive harness restart

hive_ag3nt::events::Bus replaces its in-memory VecDeque with a sqlite-
backed store at /state/hyperhive-events.sqlite (overridable via
HYPERHIVE_EVENTS_DB). emit() inserts a row; history() reads back the
most recent 2000 events. survives harness restart now — operator reload
mid-investigation no longer wipes the trail.

vacuum runs hourly (immediate first sweep): drop rows older than 7
days, then trim to 2000 newest. two-stage so a quiet agent keeps a
useful tail and a chatty one stays bounded. wired into both
hive-ag3nt and hive-m1nd via spawn_events_vacuum.

if the db open fails (e.g. no /state mount in dev), Bus runs in
no-store mode — events still broadcast, just nothing persisted.

TODO.md

@@ -29,14 +29,6 @@ Pick anything from here when relevant. Cross-cutting design notes live in
 
 - **Per-agent UI substance.** Show last N inbox messages, last turn timing,
   link back to dashboard.
-- **Delivered events history persistence.** The `events::Bus` ring
-  buffer (500 events, in-memory) backfills the terminal on page load
-  but dies on harness restart, and only ever holds the most recent
-  turn or two. Persist to sqlite (`events(agent, id, ts, kind,
-  payload_json)`) so the operator can scroll back through prior
-  turns, and so `/events/history` survives restart. Cap rows per
-  agent or auto-vacuum on age, same trade-off as the bounded broker
-  entry below.
 - **State badge: compacting + napping states.** Idle/thinking already
   ship (driven from SSE turn_start/turn_end). Add `compacting 📦` and
   `napping 😴` once the `/compact` trigger and `nap` tool exist —