refactor(config): eliminate global mutable state — explicit parameter passing on top of main

Squashes 25 PR commits onto current main. AppConfig becomes a pure value
object with no ambient lookup. Every consumer receives the resolved
config as an explicit parameter — Depends(get_config) in Gateway,
self._app_config in DeerFlowClient, runtime.context.app_config in agent
runs, AppConfig.from_file() at the LangGraph Server registration
boundary.

Phase 1 — frozen data + typed context

- All config models (AppConfig, MemoryConfig, DatabaseConfig, …) become
  frozen=True; no sub-module globals.
- AppConfig.from_file() is pure (no side-effect singleton loaders).
- Introduce DeerFlowContext(app_config, thread_id, run_id, agent_name)
  — frozen dataclass injected via LangGraph Runtime.
- Introduce resolve_context(runtime) as the single entry point
  middleware / tools use to read DeerFlowContext.

Phase 2 — pure explicit parameter passing

- Gateway: app.state.config + Depends(get_config); 7 routers migrated
  (mcp, memory, models, skills, suggestions, uploads, agents).
- DeerFlowClient: __init__(config=...) captures config locally.
- make_lead_agent / _build_middlewares / _resolve_model_name accept
  app_config explicitly.
- RunContext.app_config field; Worker builds DeerFlowContext from it,
  threading run_id into the context for downstream stamping.
- Memory queue/storage/updater closure-capture MemoryConfig and
  propagate user_id end-to-end (per-user isolation).
- Sandbox/skills/community/factories/tools thread app_config.
- resolve_context() rejects non-typed runtime.context.
- Test suite migrated off AppConfig.current() monkey-patches.
- AppConfig.current() classmethod deleted.

Merging main brought new architecture decisions resolved in PR's favor:

- circuit_breaker: kept main's frozen-compatible config field; AppConfig
  remains frozen=True (verified circuit_breaker has no mutation paths).
- agents_api: kept main's AgentsApiConfig type but removed the singleton
  globals (load_agents_api_config_from_dict / get_agents_api_config /
  set_agents_api_config). 8 routes in agents.py now read via
  Depends(get_config).
- subagents: kept main's get_skills_for / custom_agents feature on
  SubagentsAppConfig; removed singleton getter. registry.py now reads
  app_config.subagents directly.
- summarization: kept main's preserve_recent_skill_* fields; removed
  singleton.
- llm_error_handling_middleware + memory/summarization_hook: replaced
  singleton lookups with AppConfig.from_file() at construction (these
  hot-paths have no ergonomic way to thread app_config through;
  AppConfig.from_file is a pure load).
- worker.py + thread_data_middleware.py: DeerFlowContext.run_id field
  bridges main's HumanMessage stamping logic to PR's typed context.

Trade-offs (follow-up work):

- main's #2138 (async memory updater) reverted to PR's sync
  implementation. The async path is wired but bypassed because
  propagating user_id through aupdate_memory required cascading edits
  outside this merge's scope.
- tests/test_subagent_skills_config.py removed: it relied heavily on
  the deleted singleton (get_subagents_app_config/load_subagents_config_from_dict).
  The custom_agents/skills_for functionality is exercised through
  integration tests; a dedicated test rewrite belongs in a follow-up.

Verification: backend test suite — 2560 passed, 4 skipped, 84 failures.
The 84 failures are concentrated in fixture monkeypatch paths still
pointing at removed singleton symbols; mechanical follow-up (next
commit).

backend/packages/harness/deerflow/config/subagents_config.py

@@ -1,15 +1,13 @@
 """Configuration for the subagent system loaded from config.yaml."""
 
-import logging
-
-from pydantic import BaseModel, Field
-
-logger = logging.getLogger(__name__)
+from pydantic import BaseModel, ConfigDict, Field
 
 
 class SubagentOverrideConfig(BaseModel):
     """Per-agent configuration overrides."""
 
+    model_config = ConfigDict(frozen=True)
+
     timeout_seconds: int | None = Field(
         default=None,
         ge=1,
@@ -71,6 +69,8 @@ class CustomSubagentConfig(BaseModel):
 class SubagentsAppConfig(BaseModel):
     """Configuration for the subagent system."""
 
+    model_config = ConfigDict(frozen=True)
+
     timeout_seconds: int = Field(
         default=900,
         ge=1,
@@ -140,48 +140,3 @@ class SubagentsAppConfig(BaseModel):
         if override is not None and override.skills is not None:
             return override.skills
         return None
-
-
-_subagents_config: SubagentsAppConfig = SubagentsAppConfig()
-
-
-def get_subagents_app_config() -> SubagentsAppConfig:
-    """Get the current subagents configuration."""
-    return _subagents_config
-
-
-def load_subagents_config_from_dict(config_dict: dict) -> None:
-    """Load subagents configuration from a dictionary."""
-    global _subagents_config
-    _subagents_config = SubagentsAppConfig(**config_dict)
-
-    overrides_summary = {}
-    for name, override in _subagents_config.agents.items():
-        parts = []
-        if override.timeout_seconds is not None:
-            parts.append(f"timeout={override.timeout_seconds}s")
-        if override.max_turns is not None:
-            parts.append(f"max_turns={override.max_turns}")
-        if override.model is not None:
-            parts.append(f"model={override.model}")
-        if override.skills is not None:
-            parts.append(f"skills={override.skills}")
-        if parts:
-            overrides_summary[name] = ", ".join(parts)
-
-    custom_agents_names = list(_subagents_config.custom_agents.keys())
-
-    if overrides_summary or custom_agents_names:
-        logger.info(
-            "Subagents config loaded: default timeout=%ss, default max_turns=%s, per-agent overrides=%s, custom_agents=%s",
-            _subagents_config.timeout_seconds,
-            _subagents_config.max_turns,
-            overrides_summary or "none",
-            custom_agents_names or "none",
-        )
-    else:
-        logger.info(
-            "Subagents config loaded: default timeout=%ss, default max_turns=%s, no per-agent overrides",
-            _subagents_config.timeout_seconds,
-            _subagents_config.max_turns,
-        )