feat(subagents): allow model override per subagent in config.yaml (#2064)

* feat(subagents): allow model override per subagent in config.yaml

Wire the existing SubagentConfig.model field to config.yaml so users
can assign different models to different subagent types.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* test(subagents): cover model override in SubagentsAppConfig + registry

Addresses review feedback on #2064:

- registry.py: update stale inline comment — the block now applies
  timeout, max_turns AND model overrides, not just timeout.
- test_subagent_timeout_config.py: add coverage for model override
  resolution across SubagentOverrideConfig, SubagentsAppConfig
  (get_model_for + load), and registry.get_subagent_config:
  - per-agent model override is applied to registry-returned config
  - omitted `model` keeps the builtin value
  - explicit `model: null` in config.yaml is equivalent to omission
  - model override on one agent does not affect other agents
  - model override preserves all other fields (name, description,
    timeout_seconds, max_turns)
  - model override does not mutate BUILTIN_SUBAGENTS

Copilot's suggestion (3) "setting model to 'inherit' forces inheritance"
is skipped intentionally: there is no 'inherit' sentinel in the current
implementation — model is `str | None`, and None already means
"inherit from parent". Adding a sentinel would be a new feature, not
test coverage for this PR.

Tests run locally: 51 passed (37 existing + 14 new / expanded).

* test(subagents): reject empty-string model at config load time

Addresses WillemJiang's review comment on #2064 (empty-string edge case):

- subagents_config.py: add `min_length=1` to the `model` field on
  SubagentOverrideConfig. `model: ""` in config.yaml would otherwise
  bypass the `is not None` check and reach create_chat_model(name="")
  as a confusing runtime error. This is symmetric with the existing
  `ge=1` guards on timeout_seconds / max_turns, so the validation style
  stays consistent across all three override fields.
- test_subagent_timeout_config.py: add test_rejects_empty_model
  mirroring the existing test_rejects_zero / test_rejects_negative
  cases; update the docstring on test_model_accepts_any_string (now
  test_model_accepts_any_non_empty_string) to reflect the new guard.

Not addressing the first comment (validating `model` against the
`models:` section at load time) in this PR. `SubagentsAppConfig` is
scoped to the `subagents:` block and cannot see the sibling `models:`
section, so proper cross-section validation needs a second pass or a
structural change that is out of scope here — and the current behavior
is consistent with how timeout_seconds / max_turns work today. Happy to
track this as a follow-up issue covering cross-section validation
uniformly for all three fields.

Tests run locally: 52 passed in this file; 1847 passed, 18 skipped
across the full backend suite. Ruff check + format clean.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/packages/harness/deerflow/subagents/registry.py

@@ -23,7 +23,8 @@ def get_subagent_config(name: str) -> SubagentConfig | None:
     if config is None:
         return None
 
-    # Apply timeout override from config.yaml (lazy import to avoid circular deps)
+    # Apply runtime overrides (timeout, max_turns, model) from config.yaml
+    # Lazy import to avoid circular deps.
     from deerflow.config.subagents_config import get_subagents_app_config
 
     app_config = get_subagents_app_config()
@@ -47,6 +48,15 @@ def get_subagent_config(name: str) -> SubagentConfig | None:
             effective_max_turns,
         )
         overrides["max_turns"] = effective_max_turns
+    effective_model = app_config.get_model_for(name)
+    if effective_model is not None and effective_model != config.model:
+        logger.debug(
+            "Subagent '%s': model overridden by config.yaml (%s -> %s)",
+            name,
+            config.model,
+            effective_model,
+        )
+        overrides["model"] = effective_model
     if overrides:
         config = replace(config, **overrides)