Harden DeepSeek agent: LiteLLM adapter, DSML/reasoning/embeddings/error fixes

- LiteLLMAdapter (subclasses OpenAIAdapter via _acreate hook): routes DeepSeek
  through LiteLLM. Opt-in AGENTIC_DEFAULT_MODEL_PROVIDER=litellm. A/B beat the
  hand-rolled adapter (0 DSML, 0 parse-fails). Defensive chunk.usage getattr,
  token-estimate usage fallback for billing, quiet litellm logs.
- DSML parser: tolerate single/multi fullwidth pipes, honor string="true/false"
  typed args (openai_adapter fallback when DeepSeek leaks tool calls as text).
- Thinking mode: capture and round-trip reasoning_content across turns.
- Embeddings: dedicated AGENTIC_EMBEDDINGS_API_KEY (DeepSeek has no embeddings);
  disable cleanly when unset to avoid per-turn 401.
- claude_format: friendly generic error messages to the chat, raw only in logs.
- acai agent max_tokens 4096->16384 (whole-file writes no longer truncate);
  system.md size-based edit policy; strict tools opt-in (off).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

requirements.txt

@@ -5,6 +5,7 @@ pydantic-settings>=2.7.0,<3.0.0
 redis[hiredis]>=5.2.0,<6.0.0
 anthropic>=0.42.0,<1.0.0
 openai>=1.60.0,<2.0.0
+litellm>=1.50.0,<2.0.0
 httpx>=0.28.0,<1.0.0
 sse-starlette>=2.2.0,<3.0.0
 tiktoken>=0.7.0,<1.0.0