feat: time awareness — Lyra perceives 'now' and how long it's been

She had no clock: current date/time and the gap since Brian last spoke were
invisible between turns, and reflection was timeless. Now:
- lyra/clock.py: wall-clock stamp + coarse human gaps ("3 days")
- chat: inject a 'now' note (date/time + gap since last turn) after her
  self-state — when she is, before the world
- reflect(): feed current time + silence gap into reflection, neutrally —
  prompt invites her to weigh elapsed time "to whatever degree it genuinely
  affects you" (no prescribed feeling; whether silence means anything is left
  to emerge)
- memory.last_exchange_at(): timestamp of the most recent exchange

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lyra/memory.py

@@ -386,6 +386,17 @@ def profile_sessions_covered(profile_id: str = "self") -> int:
     return int(r["sessions_covered"]) if r else 0
 
 
+def last_exchange_at() -> str | None:
+    """ISO timestamp of the most recent exchange overall (None if there are none).
+
+    Used to tell Lyra how long it's been since Brian last said anything — the
+    gap she perceives between turns and while she's idle between conversations.
+    """
+    conn = _connection()
+    r = conn.execute("SELECT MAX(created_at) AS m FROM exchanges").fetchone()
+    return r["m"] if r and r["m"] else None
+
+
 def backlog_stats(ripe_threshold: int = 20) -> dict:
     """Snapshot of the consolidation backlog, for the dream cycle to sense.