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>

agent/sql/0001_overlord_agent_ro.sql

@@ -0,0 +1,35 @@
+-- Read-only PG role for the Overlord Agent's `query_telemetry_db` MCP tool.
+--
+-- This is the second line of defense (the first is the sqlglot parser in
+-- agent/tools.py:assert_read_only). Even a parser bypass cannot mutate
+-- because this role only has SELECT.
+--
+-- Apply on the dereth-db container:
+--   docker exec dereth-db psql -U postgres -d dereth -f - < agent/sql/0001_overlord_agent_ro.sql
+-- (substitute the password before running, or keep as a placeholder and
+-- ALTER ROLE … PASSWORD '…' separately)
+
+DO $$
+BEGIN
+    IF NOT EXISTS (SELECT 1 FROM pg_roles WHERE rolname = 'overlord_agent_ro') THEN
+        CREATE ROLE overlord_agent_ro NOINHERIT LOGIN PASSWORD 'change-me-set-via-alter-role';
+    END IF;
+END$$;
+
+GRANT CONNECT ON DATABASE dereth TO overlord_agent_ro;
+GRANT USAGE ON SCHEMA public TO overlord_agent_ro;
+
+-- Grant SELECT on all current public tables.
+GRANT SELECT ON ALL TABLES IN SCHEMA public TO overlord_agent_ro;
+GRANT SELECT ON ALL SEQUENCES IN SCHEMA public TO overlord_agent_ro;
+
+-- And on any future tables created in public.
+ALTER DEFAULT PRIVILEGES IN SCHEMA public
+    GRANT SELECT ON TABLES TO overlord_agent_ro;
+
+-- TimescaleDB-internal schema (chunks live here). Read on hypertable chunks
+-- requires SELECT on _timescaledb_internal too.
+GRANT USAGE ON SCHEMA _timescaledb_internal TO overlord_agent_ro;
+GRANT SELECT ON ALL TABLES IN SCHEMA _timescaledb_internal TO overlord_agent_ro;
+ALTER DEFAULT PRIVILEGES IN SCHEMA _timescaledb_internal
+    GRANT SELECT ON TABLES TO overlord_agent_ro;