fix(dispatch): presence is three-state — absent status key is unknown, not off-duty

The previous commit treated a missing miler_status:<id> key as "not available".
In production only 2 of 34 milers have that key at all, so the agent would have
reported "no available rider" for nearly every failure — a confident wrong
answer in the opposite direction from the bug it fixed.

- _miler_presence returns available / unavailable / unknown. No key, an
  unparseable value, or an unrecognised status reads as unknown.
- _find_zone prefers a confirmed-available miler, otherwise reports the nearest
  unknown-presence one (it may well be assignable), and returns None only when
  every nearby candidate is confirmed off duty.
- Facts carry nearest_miler_within_km + nearest_miler_presence; the prompt
  states plainly that unknown presence is not evidence of a coverage gap and
  should lean to monitor/escalate rather than ops_alert.
- New eval case for riders-nearby-but-no-presence-data; tests for all three
  presence states.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012AJLYcbTHCe45fyFnMfEin

evals/assignment_eval.py

@@ -4,7 +4,7 @@ Offline eval for the DispatchAgent assignment-failure decision
 
 Same scoring as the stall eval (see evals/_harness.py): acceptable-rate is the
 headline, exact-rate secondary. The cases use the real gatherer keys
-(zone_id, failures_today, nearest_available_miler_within_km,
+(zone_id, failures_today, nearest_miler_within_km, nearest_miler_presence,
 milers_in_geo_index_within_30km, has_coordinates) so the eval
 reflects what DispatchAgent actually sends.