feat(boards): Store gateway config per board

Move gateway configuration into board settings and wire agent\nprovisioning, heartbeat templates, and gateway status lookups\nto use board-specific gateway settings. Adds board_id on agents\nand UI updates for board-scoped selection.\n\nCo-Authored-By: Claude <noreply@anthropic.com>

templates/HEARTBEAT.md

@@ -6,6 +6,7 @@ If this file is empty, skip heartbeat work.
 - BASE_URL (e.g. http://localhost:8000)
 - AUTH_TOKEN (agent token)
 - AGENT_NAME
+- BOARD_ID
 
 ## Schedule
 - Run this heartbeat every 10 minutes.
@@ -17,7 +18,7 @@ If this file is empty, skip heartbeat work.
 curl -s -X POST "$BASE_URL/api/v1/agents/heartbeat" \
   -H "X-Agent-Token: $AUTH_TOKEN" \
   -H "Content-Type: application/json" \
-  -d '{"name": "'$AGENT_NAME'", "status": "online"}'
+  -d '{"name": "'$AGENT_NAME'", "board_id": "'$BOARD_ID'", "status": "online"}'
 ```
 
 2) List boards:

templates/TOOLS.md

@@ -6,6 +6,7 @@ MAIN_SESSION_KEY={{ main_session_key }}
 WORKSPACE_ROOT={{ workspace_root }}
 AGENT_NAME={{ agent_name }}
 AGENT_ID={{ agent_id }}
+BOARD_ID={{ board_id }}
 SESSION_KEY={{ session_key }}
 WORKSPACE_PATH={{ workspace_path }}