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/conftest.py

@@ -0,0 +1,33 @@
+"""Test configuration for the backend test suite.
+
+Sets up sys.path and pre-mocks modules that would cause circular import
+issues when unit-testing lightweight config/registry code in isolation.
+"""
+
+import sys
+from pathlib import Path
+from unittest.mock import MagicMock
+
+# Make 'src' importable from any working directory
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+# Break the circular import chain that exists in production code:
+#   src.subagents.__init__
+#     -> .executor (SubagentExecutor, SubagentResult)
+#       -> src.agents.thread_state
+#         -> src.agents.__init__
+#           -> lead_agent.agent
+#             -> subagent_limit_middleware
+#               -> src.subagents.executor  <-- circular!
+#
+# By injecting a mock for src.subagents.executor *before* any test module
+# triggers the import, __init__.py's "from .executor import ..." succeeds
+# immediately without running the real executor module.
+_executor_mock = MagicMock()
+_executor_mock.SubagentExecutor = MagicMock
+_executor_mock.SubagentResult = MagicMock
+_executor_mock.SubagentStatus = MagicMock
+_executor_mock.MAX_CONCURRENT_SUBAGENTS = 3
+_executor_mock.get_background_task_result = MagicMock()
+
+sys.modules["src.subagents.executor"] = _executor_mock