0.16.6

## Highlights
- Expanded Conversations API support for **default conversation / agent-direct mode**.
- New conversations now initialize with a **compiled system message at creation time**.
- Fixed `model_settings.max_output_tokens` default behavior so it **does not silently override** existing `max_tokens` unless explicitly set.

## Conversations API updates
- Added support for `conversation_id="default"` + `agent_id` across conversation endpoints (send/list/cancel/compact/stream retrieve).
- Kept backwards compatibility for `conversation_id=agent-*` (deprecated path).
- Added lock-key handling in agent-direct flows to avoid concurrent execution conflicts.

## Conversation/system-message behavior
- Conversation creation now compiles and persists a system message immediately.
- This captures current memory state at conversation start and removes first-message timing edge cases.

## Model/config updates
- Added model support for:
  - `gpt-5.3-codex`
  - `gpt-5.3-chat-latest`
- Updated defaults:
  - context window default: **32k → 128k**
  - `CORE_MEMORY_BLOCK_CHAR_LIMIT`: **20k → 100k**
- Anthropic model settings now allow `effort="max"` where supported.
- Gemini request timeout default increased to **600s**.

## Memory / memfs updates
- Git-backed memory frontmatter no longer emits `limit` (legacy `limit` keys are removed on merge).
- Skills sync now maps only `skills/{name}/SKILL.md` to `skills/{name}` block labels.
- Other markdown under `skills/` is intentionally ignored for block sync.
- Memory filesystem rendering now includes descriptions for non-`system/` files and condenses skill display.

## Reliability and compatibility fixes
- Added explicit `LLMEmptyResponseError` handling for empty Anthropic streaming responses.
- Improved Fireworks compatibility by stripping unsupported reasoning fields.
- Improved Z.ai compatibility by mapping `max_completion_tokens` to `max_tokens`.

**Full Changelog**: https://github.com/letta-ai/letta/compare/0.16.5...0.16.6