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/auth.py

@@ -0,0 +1,51 @@
+"""Session-cookie validation that mirrors main.py.
+
+Re-implements the verify path so this host-side service can authenticate
+the same browser cookie that dereth-tracker issues. Both services must
+share the SECRET_KEY env var.
+"""
+
+from __future__ import annotations
+
+import os
+
+from fastapi import HTTPException, Request, status
+from itsdangerous import BadSignature, SignatureExpired, URLSafeTimedSerializer
+
+# Mirror main.py:996-998
+SECRET_KEY = os.getenv("SECRET_KEY", "change-me-in-production-please")
+SESSION_MAX_AGE = 30 * 24 * 3600  # 30 days
+_serializer = URLSafeTimedSerializer(SECRET_KEY)
+
+
+def verify_session_cookie(token: str) -> dict | None:
+    """Verify and decode a session token. Returns None if invalid/expired.
+
+    Mirrors main.py:1013-1019 byte-for-byte so a cookie issued by the tracker
+    decodes here identically.
+    """
+    try:
+        data = _serializer.loads(token, max_age=SESSION_MAX_AGE)
+        return {"username": data["u"], "is_admin": data["a"]}
+    except (BadSignature, SignatureExpired, KeyError):
+        return None
+
+
+def require_user(request: Request) -> dict:
+    """FastAPI dependency: enforces a valid session cookie.
+
+    Returns the decoded user dict on success; raises 401 otherwise.
+    """
+    token = request.cookies.get("session")
+    if not token:
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail="Not authenticated",
+        )
+    user = verify_session_cookie(token)
+    if not user:
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail="Session invalid or expired",
+        )
+    return user