feat(subagents): make subagent timeout configurable via config.yaml (#897)

* feat(subagents): make subagent timeout configurable via config.yaml

- Add SubagentsAppConfig supporting global and per-agent timeout_seconds
- Load subagents config section in AppConfig.from_file()
- Registry now applies config.yaml overrides without mutating builtin defaults
- Polling safety-net in task_tool is now dynamic (execution timeout + 60s buffer)
- Document subagents section in config.example.yaml
- Add make test command and enforce TDD policy in CLAUDE.md
- Add 38 unit tests covering config validation, timeout resolution, registry
  override behavior, and polling timeout formula

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* feat(subagents): add logging for subagent timeout config and execution

- Log loaded timeout config (global default + per-agent overrides) on startup
- Log debug message in registry when config.yaml overrides a builtin timeout
- Include timeout in executor's async execution start log
- Log effective timeout and polling limit when a task is dispatched
- Fix UnboundLocalError: move max_poll_count assignment before logger.info

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* ci(backend): add lint step and run all unit tests via Makefile

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix lint

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/subagents/executor.py

@@ -343,7 +343,7 @@ class SubagentExecutor:
             status=SubagentStatus.PENDING,
         )
 
-        logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} starting async execution, task_id={task_id}")
+        logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} starting async execution, task_id={task_id}, timeout={self.config.timeout_seconds}s")
 
         with _background_tasks_lock:
             _background_tasks[task_id] = result