agentd: add updated_at timestamp to status JSON

Goal

Enable pull-based notification workflow: on heartbeat, Ava runs agentd status --json | jq filtered/sorted by a timestamp that reflects the last time an agent's state changed, so we can detect 'new idle' vs 'already relayed idle' without a push notification subsystem.

Context

Ben's design (see recent Telegram thread with Ava): the agentd notification channel should be pull-based, not push-based. The heartbeat pattern will be something like:

agentd status --json | jq '.[] | select(.status == "idle") | select(.updated_at > $LAST_CHECK)' 

No need for mail/webhook subsystem.

What's missing

AgentInfo (intent/Omni/Agentd/Daemon.hs:224) currently has aiStartedAt and aiCompletedAt but no general 'last state change' timestamp. A long-running persistent agent that transitions running→idle→running→idle only updates started_at once and completed_at never (it doesn't complete), so there's no way to tell *when* it became idle.

Acceptance criteria

1. Add aiUpdatedAt :: Maybe Time.UTCTime field to AgentInfo in intent/Omni/Agentd/Daemon.hs (near aiStartedAt). 2. Add updated_at TEXT column to the agents table schema (intent/Omni/Agentd/Daemon.hs initDb ~line 346). Include a schema migration: ALTER TABLE agents ADD COLUMN updated_at TEXT guarded so re-running is safe. 3. Emit "updated_at" .= aiUpdatedAt info in the ToJSON instance (~line 285). 4. Update every UPDATE agents SET ... statement (lines 418, 425, 432, 440, 447, 454, 461, 468) to also set updated_at = ? with the current time. Easiest approach: add a helper touchUpdated :: SQL.Connection -> Text -> IO () and call it at every status/summary transition, OR add updated_at = ? to each existing UPDATE (and pass now as an extra parameter). 5. SELECT queries that build AgentInfo must populate aiUpdatedAt from the column. 6. Verify: spawn a persistent agent, let it go idle, confirm agentd status --json shows an updated_at that changes on each turn_end.

Out of scope

• No new CLI flag on agentd status — we're using jq for filtering.
• No changes to webhook/notification code (that's being deprecated).
• No schema cleanup of started_at/completed_at (keep them).

Testing

• Unit test (if there's a JSON encoding test) that updated_at serializes.
• Manual: agentd run -n test-idle 'say hi then wait' → wait for idle → agentd status --json | jq '.[] | select(.run_id == "test-idle") | .updated_at' should be populated and more recent than started_at.