refactor: centralize Gemini default-off reasoning in reasoning_params

Follow-up to #490. The PR inlined the "disable thinking for Gemini 2.5/3"
gate in 5 places across 3 files. This commit collapses them to one
registry and three thin call sites.

Changes:

* `services/llm/reasoning_params.py`: new `_PROVIDER_DEFAULT_OFF_PATTERNS`
  registry + `default_reasoning_effort_for(provider, model)` public helper.
  `build_openai_compatible_reasoning_kwargs` now consults the registry, so
  the openai-compat path (which lost its inline gate during the #490 merge
  conflict resolution) is restored via the helper.

* `services/llm/executors.py` (sdk_complete + sdk_stream) and
  `services/llm/cloud_provider.py` (_openai_complete + _openai_stream) now
  call `default_reasoning_effort_for(...)` instead of inlining the
  ('gemini-2.5', 'gemini-3') startswith check.

* Use substring (not startswith) match so `models/gemini-2.5-flash` is
  also covered — some OpenAI-compat clients prefix model ids with `models/`.

* `services/config/loader.py:get_agent_params`: when a module's section is
  missing from the user's stale `agents.yaml`, fall back through
  `DEFAULT_AGENTS_SETTINGS` before the global `(0.5, 4096)` default. This
  lets the `capabilities.visualize` default (`max_tokens=16384` from #490)
  reach existing installs, not just fresh ones.

* `capabilities/visualize.py`: hoist the duplicated lazy
  `from deeptutor.agents.visualize.models import ReviewResult` import
  from two branches into the top of `run()`.

Tests:

* `tests/services/llm/test_reasoning_params.py` — 17 new cases covering
  Gemini 2.5/3 + `models/` prefix + case-insensitivity + the legacy
  Gemini 1.5/2.0 / other-provider untouched paths + the explicit-override
  takes-precedence rule.
* All 107 tests in `tests/services/llm/` still pass; 5 pre-existing
  `test_chat_params_config` failures (8192 vs 8000 drift) are unrelated.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>