refactor: dedupe inner-tool model routing into a single helper

[adamlu123]

Follow-up to #5. Same behavior (Anthropic-only runs work), but smaller surface.

What changed

• New tools/_model_config.py consolidates the model-config helpers that
  PR fix: route image_qa and self_reflection through the configured model (closes #3) #5 added verbatim to both image_qa.py and self_reflection.py.
  Exposes load_tool_model, resolve_model_config_path, and
  _extract_model_block.
• Path resolution is now two-step: explicit --model-config arg, then
  <workspace_dir>/config_snapshot/merged_config.yaml (already written
  by cli.py). Dropped the WEBWRIGHT_TOOL_MODEL_CONFIG env var and the
  tool_model_config.json fallback — neither was produced by anything
  in the repo.
• Removed the legacy OpenAI direct-HTTP code paths from both tools
  (_call_openai, _openai_config, _post_with_retry, _sleep_backoff,
  _RETRYABLE_STATUS_CODES) and the CLI flags they relied on
  (--api-key, --endpoint, --model, --max-attempts,
  --retry-base-delay). The model client owns HTTP retries via
  models/base.py:_post_with_retries.
• model_claude.yaml no longer declares the &model YAML anchor or the
  tools.image_qa.model / tools.self_reflection.model block — the
  tools read the top-level model: block directly.

Tests

tests/unit/test_tool_model_routing.py now covers:

• model_claude.yaml sets a top-level Anthropic model
• model_claude.yaml has no tools: block
• _extract_model_block reads the top-level model / raises if missing
• resolve_model_config_path honors an explicit arg, falls back to the
  snapshot, and raises FileNotFoundError when neither exists

All 7 pass.

Behavior note

The --api-key/--endpoint/--model direct CLI invocation paths are
gone. Callers must either pass --model-config <path> or run via the
agent (which writes config_snapshot/merged_config.yaml).

Net: -583 / +106 lines vs. the state after #5 merged.