chore(config): delete 51 dead keys, fix usgs_quake floor, secret-flag consistency (#46)

Backend half of making the dashboard the complete config surface (per an
exhaustive per-key audit).

Delete 51 vestigial/unread config fields (load-safe: _dict_to_dataclass
whitelists by field, so existing files carrying these keys still load and the
keys drop on next save):
- 36 duplicated MQTT block (host/port/username/password/topic_root/use_tls)
  on memory/context/commands + env nws/swpc/ducting (grep-proven unread;
  mesh_sources keeps its real MQTT fields)
- 3 no-op history cleanup keys (auto_cleanup/cleanup_interval_hours/max_age_days)
- 5 alert scaffolding (alert_cooldown_minutes, RegionAnchor.nws_zones,
  battery_{warning,critical,emergency}_voltage)
- 5 danger-zone non-fire min_acres (kept fire.min_acres via a fire subclass)
- 2 deprecated adapter_config keys (nws.broadcast_severities/warning_suffix_promotes)

Bug: usgs_quake native magnitude floor was unreachable from the GUI (native
reads config.min_magnitude; the GUI "Global Floor" wrote the registry
global_mag_floor that only the Central path reads). Reconciled: min_magnitude
is the canonical native floor the frontend will bind; registry floors marked
Central-path-only. Effective filtering unchanged.

Secret-flag consistency: add environmental.roads511.api_key + wzdx.api_key to
SECRET_FIELDS (secrets move to .env in the follow-up; ${VAR} interpolation kept).

Suite at 10-failure baseline (1703 passed).

Co-authored-by: Matt Johnson <mj@k7zvx.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

work/config.example.yaml

@@ -30,9 +30,6 @@ history:
   database: /data/conversations.db
   max_messages_per_user: 50      # Messages to keep per user
   conversation_timeout: 86400    # Conversation expiry (seconds, 86400=24h)
-  auto_cleanup: true             # Auto-delete old conversations
-  cleanup_interval_hours: 24     # How often to run cleanup
-  max_age_days: 30               # Delete conversations older than this
 
 # === MEMORY OPTIMIZATION ===
 memory: