feat(agent): Phase 1 — chat-window AI assistant via Claude Code subprocess

Adds an in-dashboard AI assistant that answers questions about live game
state. Designed reactively (no background loops) — every user message in
the chat window or via /api/agent/ask runs one `claude -p` invocation.

Architecture:
- New host-side FastAPI service (agent/) on 127.0.0.1:8767, OUTSIDE the
  dereth-tracker Docker container because `claude` and ~/.claude
  credentials live on the host.
- nginx routes /api/agent/* to the host service.
- The same browser session cookie the tracker issues authenticates
  agent requests (shared SECRET_KEY).
- The agent shells out to `claude -p --session-id <uuid>` with
  cwd=/home/erik/MosswartOverlord. Sessions persist as JSONL on disk
  via Claude Code's built-in machinery.
- An MCP stdio server (agent/mcp_overlord.py) exposes tools to Claude:
  get_live_players, get_recent_rares, query_telemetry_db (read-only,
  parsed by sqlglot to reject DML/DDL), get_player_state, get_inventory,
  get_inventory_search, get_combat_stats, get_equipment_cantrips,
  get_quest_status, get_server_health, suitbuilder_search.
- Read-only PG role (overlord_agent_ro) is the second line of defense
  on the SQL tool — even a parser bypass can't mutate.

Frontend:
- AgentWindow.tsx — draggable chat window with localStorage-pinned
  session UUID, "New Chat" button, on-mount rehydration from
  /agent/sessions/{id}/history (parses Claude Code's JSONL).
- Wired into WindowRenderer + Sidebar (🤖 Assistant button).

Operational:
- systemd unit (overlord-agent.service) + install.sh.
- agent/README.md documents env vars, deploy flow, smoke tests.
- nginx/overlord.conf gets a new /api/agent/ location with 180s timeout.
- CLAUDE.md gets an "Overlord Assistant Mode" section briefing the
  agent on which tools to use and how to behave.

NOT YET DEPLOYED — server still needs:
1. Apply agent/sql/0001_overlord_agent_ro.sql + ALTER ROLE password
2. Add AGENT_DB_DSN to /home/erik/MosswartOverlord/.env
3. bash agent/install.sh (creates venv, installs unit, starts service)
4. sudo cp /home/erik/MosswartOverlord/nginx/overlord.conf to nginx + reload

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

nginx/overlord.conf

@@ -52,6 +52,22 @@ server {
         proxy_send_timeout     1d;
     }
 
+    # Overlord Agent — host-side service running OUTSIDE the Docker stack
+    # because it shells out to `claude` which depends on host-side
+    # ~/.claude credentials. Long timeout because agent calls can spin
+    # while Claude Code chains tool invocations.
+    location /api/agent/ {
+        proxy_pass           http://127.0.0.1:8767/agent/;
+        proxy_http_version   1.1;
+        proxy_set_header     Host              $host;
+        proxy_set_header     X-Real-IP         $remote_addr;
+        proxy_set_header     X-Forwarded-For   $proxy_add_x_forwarded_for;
+        proxy_set_header     X-Forwarded-Proto $scheme;
+        proxy_pass_request_headers on;
+        proxy_read_timeout   180s;
+        proxy_send_timeout   180s;
+    }
+
     # API endpoints (live, trails, history, stats) — short-lived HTTP
     location /api/ {
         proxy_pass           http://tracker/;