refactor(ai_client): Remove 12 module-level _X_history aliases

Phase 7 of code_path_audit_phase_3_provider_state_20260624.
Per-provider history is now accessed via provider_state.get_history()
at call sites; the 12 module-level _X_history/_X_history_lock aliases
are no longer referenced anywhere in production code (helper function
DEFINITIONS that take history as a parameter are unaffected).

src/ai_client.py

@@ -110,29 +110,17 @@ _gemini_cached_file_paths: list[str]       = []
 _GEMINI_CACHE_TTL: int = 3600
 
 _anthropic_client:       Optional[anthropic.Anthropic] = None
-_anthropic_history = provider_state.get_history("anthropic")
-_anthropic_history_lock = _anthropic_history.lock
 
 _deepseek_client:       Any                  = None
-_deepseek_history = provider_state.get_history("deepseek")
-_deepseek_history_lock = _deepseek_history.lock
 
 _minimax_client:       Any                  = None
-_minimax_history = provider_state.get_history("minimax")
-_minimax_history_lock = _minimax_history.lock
 
 _qwen_client:       Any                  = None
-_qwen_history = provider_state.get_history("qwen")
-_qwen_history_lock = _qwen_history.lock
 _qwen_region:       str            = "china"
 
 _grok_client:       Any                  = None
-_grok_history = provider_state.get_history("grok")
-_grok_history_lock = _grok_history.lock
 
 _llama_client:       Any                  = None
-_llama_history = provider_state.get_history("llama")
-_llama_history_lock = _llama_history.lock
 _llama_base_url:     str = "http://localhost:11434/v1"
 _llama_api_key:      str = "ollama"