feat: Complete Bannerlord MCP Harness implementation (Issue #722)

Implements the Hermes observation/control path for local Bannerlord per GamePortal Protocol.

## New Components

- nexus/bannerlord_harness.py (874 lines)
  - MCPClient for JSON-RPC communication with MCP servers
  - capture_state() → GameState with visual + Steam context
  - execute_action() → ActionResult for all input types
  - observe-decide-act loop with telemetry through Hermes WS
  - Bannerlord-specific actions (inventory, party, save/load)
  - Mock mode for testing without game running

- mcp_servers/desktop_control_server.py (14KB)
  - 13 desktop automation tools via pyautogui
  - Screenshot, mouse, keyboard control
  - Headless environment support

- mcp_servers/steam_info_server.py (18KB)
  - 6 Steam Web API tools
  - Mock mode without API key, live mode with STEAM_API_KEY

- tests/test_bannerlord_harness.py (37 tests, all passing)
  - GameState/ActionResult validation
  - Mock mode action tests
  - ODA loop tests
  - GamePortal Protocol compliance tests

- docs/BANNERLORD_HARNESS_PROOF.md
  - Architecture documentation
  - Proof of ODA loop execution
  - Telemetry flow diagrams

- examples/harness_demo.py
  - Runnable demo showing full ODA loop

## Updates

- portals.json: Bannerlord metadata per GAMEPORTAL_PROTOCOL.md
  - status: active, portal_type: game-world
  - app_id: 261550, window_title: 'Mount & Blade II: Bannerlord'
  - telemetry_source: hermes-harness:bannerlord

## Verification

pytest tests/test_bannerlord_harness.py -v
37 passed, 2 skipped, 11 warnings

Closes #722

portals.json

@@ -17,13 +17,23 @@
     "id": "bannerlord",
     "name": "Bannerlord",
     "description": "Calradia battle harness. Massive armies, tactical command.",
-    "status": "standby",
+    "status": "active",
     "color": "#ffd700",
     "position": { "x": -15, "y": 0, "z": -10 },
     "rotation": { "y": 0.5 },
+    "portal_type": "game-world",
+    "world_category": "strategy-rpg",
+    "environment": "production",
+    "access_mode": "operator",
+    "readiness_state": "active",
+    "telemetry_source": "hermes-harness:bannerlord",
+    "owner": "Timmy",
+    "app_id": 261550,
+    "window_title": "Mount & Blade II: Bannerlord",
     "destination": {
       "url": "https://bannerlord.timmy.foundation",
       "type": "harness",
+      "action_label": "Enter Calradia",
       "params": { "world": "calradia" }
     }
   },