fix(mcp): replace deprecated status param with state_group in task queries

- tasks.list: replace status filter with state_group (backlog/unstarted/started/completed/cancelled)
- tasks.create: remove obsolete status param (tasks now use state_id FK)
- tasks.update: remove obsolete status param
- Implement state_group filtering via EXISTS subquery on states table
- Update API.md example to use state_group instead of status

docs/API.md

@@ -1046,7 +1046,7 @@ Requests and responses use the JSON-RPC 2.0 envelope:
 {
   "jsonrpc": "2.0",
   "method": "tools/call",
-  "params": { "name": "tasks.list", "arguments": { "domain_id": "b2c3d4e5-...", "status": "todo" } },
+  "params": { "name": "tasks.list", "arguments": { "domain_id": "b2c3d4e5-...", "state_group": "unstarted" } },
   "id": 1
 }
 ```