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/tests/test_docker_sandbox_mode_detection.py

@@ -16,11 +16,7 @@ def _detect_mode_with_config(config_content: str) -> str:
         tmp_root = Path(tmpdir)
         (tmp_root / "config.yaml").write_text(config_content)
 
-        command = (
-            f"source '{SCRIPT_PATH}' && "
-            f"PROJECT_ROOT='{tmp_root}' && "
-            "detect_sandbox_mode"
-        )
+        command = f"source '{SCRIPT_PATH}' && PROJECT_ROOT='{tmp_root}' && detect_sandbox_mode"
 
         output = subprocess.check_output(
             ["bash", "-lc", command],
@@ -33,11 +29,7 @@ def _detect_mode_with_config(config_content: str) -> str:
 def test_detect_mode_defaults_to_local_when_config_missing():
     """No config file should default to local mode."""
     with tempfile.TemporaryDirectory() as tmpdir:
-        command = (
-            f"source '{SCRIPT_PATH}' && "
-            f"PROJECT_ROOT='{tmpdir}' && "
-            "detect_sandbox_mode"
-        )
+        command = f"source '{SCRIPT_PATH}' && PROJECT_ROOT='{tmpdir}' && detect_sandbox_mode"
         output = subprocess.check_output(["bash", "-lc", command], text=True).strip()
 
     assert output == "local"