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/README.md

@@ -0,0 +1,146 @@
+# Overlord Agent
+
+A small host-side Python service that gives Claude Code (running in
+headless mode) access to live Overlord data so it can answer questions
+from the dashboard chat window.
+
+## Why a separate service?
+
+`dereth-tracker` runs in Docker. The `claude` CLI binary at
+`/home/erik/.local/bin/claude` depends on `~/.claude` credentials owned
+by user `erik` on the host. The tracker container can't invoke it.
+
+So this service runs **outside** Docker, listens on `127.0.0.1:8767`,
+and nginx routes `/api/agent/*` to it. It validates the same browser
+session cookie the tracker issues (shared `SECRET_KEY`) and shells out
+to `claude -p` with `cwd=/home/erik/MosswartOverlord`.
+
+## Architecture
+
+```
+Browser ──nginx──┬─► /api/*       ──► dereth-tracker (Docker, 8765)
+                 │
+                 └─► /api/agent/* ──► overlord-agent  (host, 8767)
+                                          │
+                                          ├─► subprocess: claude -p ...
+                                          │       │
+                                          │       └─► MCP stdio ──► mcp_overlord.py
+                                          │                              │
+                                          │                              └─► HTTP loopback to tracker
+                                          │                              └─► asyncpg to dereth-db
+                                          │
+                                          └─► validates "session" cookie
+```
+
+## Files
+
+| File | What |
+|------|------|
+| `service.py` | FastAPI app (`/agent/health`, `/agent/sessions/new`, `/agent/ask`, `/agent/sessions/{id}/history`) |
+| `auth.py` | Session-cookie validation (mirrors `main.py:1013-1019`) |
+| `claude_wrapper.py` | `asyncio.create_subprocess_exec("claude", "-p", ...)` |
+| `tools.py` | Pure tool implementations (HTTP loopback + read-only DB) |
+| `mcp_overlord.py` | MCP stdio server registering tools for Claude Code |
+| `sql/0001_overlord_agent_ro.sql` | Read-only PG role for the SQL tool |
+| `overlord-agent.service` | systemd unit |
+| `install.sh` | One-shot installer (venv + pip install + systemd) |
+
+## Required env vars (in repo-root `.env`)
+
+```
+SECRET_KEY=<same value the tracker uses to sign cookies>
+AGENT_DB_DSN=postgresql://overlord_agent_ro:<password>@127.0.0.1:5432/dereth
+TRACKER_URL=http://127.0.0.1:8765           # optional, this is the default
+CLAUDE_BIN=/home/erik/.local/bin/claude     # optional, this is the default
+CLAUDE_CWD=/home/erik/MosswartOverlord      # optional, this is the default
+CLAUDE_TIMEOUT_S=120                        # optional
+```
+
+## First-time setup on the server
+
+1. **Create the read-only DB role** (one-time):
+   ```bash
+   docker exec -i dereth-db psql -U postgres -d dereth \
+       < /home/erik/MosswartOverlord/agent/sql/0001_overlord_agent_ro.sql
+   docker exec -it dereth-db psql -U postgres -d dereth \
+       -c "ALTER ROLE overlord_agent_ro PASSWORD '<random-password>';"
+   ```
+2. **Add `AGENT_DB_DSN`** to `/home/erik/MosswartOverlord/.env` with the
+   password you just set.
+3. **Run the installer**:
+   ```bash
+   cd /home/erik/MosswartOverlord
+   bash agent/install.sh
+   ```
+4. **Update nginx**: edit `/etc/nginx/sites-enabled/overlord` to add the
+   `/api/agent/` location (already in `nginx/overlord.conf` in the repo —
+   just `sudo cp` and reload).
+
+## Day-to-day deploy
+
+After editing any agent file:
+
+```bash
+# On dev:
+git push
+
+# On server:
+ssh erik@overlord.snakedesert.se
+cd /home/erik/MosswartOverlord
+git pull
+sudo systemctl restart overlord-agent
+journalctl -u overlord-agent -f   # tail logs
+```
+
+For Python dependency changes:
+
+```bash
+agent/.venv/bin/pip install -r agent/requirements.txt
+sudo systemctl restart overlord-agent
+```
+
+## Smoke tests
+
+```bash
+# 1. Service alive?
+curl http://127.0.0.1:8767/agent/health
+
+# 2. Cookie required?
+curl -X POST http://127.0.0.1:8767/agent/ask \
+    -H 'Content-Type: application/json' \
+    -d '{"session_id":"x","message":"hi"}'
+#  ⇒ 401
+
+# 3. Direct claude invocation works?
+echo "hello" | /home/erik/.local/bin/claude -p \
+    --session-id 11111111-1111-1111-1111-111111111111 \
+    --output-format json
+
+# 4. End-to-end via nginx (with cookie):
+curl -X POST https://overlord.snakedesert.se/api/agent/ask \
+    -b 'session=<your-session-cookie>' \
+    -H 'Content-Type: application/json' \
+    -d '{"session_id":"<uuid>","message":"How many characters are online?"}'
+```
+
+## Cost / rate-limit notes
+
+- Each `/agent/ask` shells out to `claude -p` once.
+- We use the user's Claude subscription (no API key) — flat-rate, no
+  per-call billing, but subscription-tier rate limits still apply.
+- **Reactive only**: there are no background loops or periodic ticks.
+  Each user message = one Claude turn (which may chain several tool
+  calls internally before producing a final answer).
+- The SQL tool is hard-capped at 10s and 200 rows.
+- `suitbuilder_search` is the only tool that can take minutes; nginx
+  read timeout is 180s for `/api/agent/`.
+
+## Adding a new MCP tool
+
+1. Implement `async def my_tool(...) -> dict` in `tools.py`.
+2. Register it in `mcp_overlord.py` under `TOOL_DEFS`:
+   - description (the agent reads this to decide when to call)
+   - JSON schema for arguments
+   - lambda dispatching to `T.my_tool(...)`
+3. `sudo systemctl restart overlord-agent`. Claude Code re-discovers the
+   tool list on each invocation.