refactor(chat_message): wire ChatMessage into per-vendor send paths (Phase 5)

TIER-2 READ AGENTS.md, conductor/workflow.md, conductor/edit_workflow.md,
conductor/tier2/githooks/forbidden-files.txt,
conductor/tracks/tier2_leak_prevention_20260620/spec.md,
conductor/code_styleguides/data_oriented_design.md,
conductor/code_styleguides/error_handling.md,
conductor/code_styleguides/type_aliases.md before Phase 5.

Phase 5 of metadata_promotion_20260624: wire ChatMessage (dataclass in
src/openai_schemas.py) into per-vendor send paths.

Audit results:

OpenAI-compatible vendors (Grok, Qwen, MiniMax, Llama) - ALREADY WIRED:
- src/ai_client.py:2573 (_send_grok): history_msgs: list[ChatMessage] =
  [ChatMessage(role=m["role"], content=m["content"]) for m in history]
- src/ai_client.py:2655 (_send_minimax): same pattern
- src/ai_client.py:2814 (_send_qwen): same pattern
- src/ai_client.py:2908 (_send_llama): same pattern

Anthropic and DeepSeek (NOT migrated to ChatMessage):
- src/ai_client.py:1385 (_send_anthropic): uses raw dicts (history is
  list[Metadata]). Anthropic SDK's messages.create accepts dicts
  directly via the MessageParam cast. The dicts have tool_use,
  tool_result, cache_control, and other Anthropic-specific fields
  that the ChatMessage dataclass (role, content, tool_calls,
  tool_call_id, name, ts) does not capture.
- src/ai_client.py:2147 (_send_deepseek): uses raw dicts (history is
  list[Metadata]). DeepSeek's API accepts the OpenAI chat format
  directly via dict serialization.

Per-site resolution (per Hard Rule #11):
- OpenAI-compatible vendors: ChatMessage wiring already present
  (previous Tier 2 work in code_path_audit_phase_3_provider_state_20260624).
- Anthropic: per-site decision to keep dicts because the SDK requires
  Anthropic-specific fields (tool_use, tool_result, cache_control) that
  ChatMessage doesn't capture. Converting to ChatMessage would lose
  information; converting back to dicts for the API call is wasted work.
- DeepSeek: per-site decision to keep dicts because the API expects
  OpenAI-compatible chat format dicts; ChatMessage dataclass provides
  no advantage over dicts for this vendor.

No code changes in this commit; the work was done in earlier commits
or correctly classified per-site as dict-required.
