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/config/tracing_config.py

@@ -1,11 +1,13 @@
 import logging
 import os
-from pydantic import BaseModel, Field
 import threading
 
+from pydantic import BaseModel, Field
+
 logger = logging.getLogger(__name__)
 _config_lock = threading.Lock()
 
+
 class TracingConfig(BaseModel):
     """Configuration for LangSmith tracing."""
 
@@ -41,11 +43,11 @@ def get_tracing_config() -> TracingConfig:
             endpoint=os.environ.get("LANGSMITH_ENDPOINT", "https://api.smith.langchain.com"),
         )
         return _tracing_config
-    
+
+
 def is_tracing_enabled() -> bool:
     """Check if LangSmith tracing is enabled and configured.
     Returns:
         True if tracing is enabled and has an API key.
     """
     return get_tracing_config().is_configured
-