v0.64.0: Session archiving, inline rename, custom model reasoning effort, and Mission Mode improvements

New features

- Session archiving - Archive sessions from the CLI to keep your session list clean
- Inline session rename - Rename sessions inline with cloud sync support
- Custom model reasoning effort - Added reasoning effort support for custom models
- Concurrent Missions warnings - Warns on Missions entry and confirms concurrent Missions runs
- Dry-run validation - New dry-run validation approach during Missions planning
- /model scope clarity - Clarified /model command scope and added set-as-default action

Bug fixes

- Mission Mode commands - Split Mission Mode enter and exit into separate commands for improved reliability
- Context management - Fixed context management triggering too frequently for OpenAI models
- OpenAI reasoning effort - Fixed maximum reasoning effort setting for OpenAI models
- Gemini rate limiting - Improved Gemini 429 error handling with throttle-aware retry
- Bedrock reasoning effort - Fixed reasoning effort settings for Bedrock models
- Empty end_turn responses - No longer treats empty end_turn responses as errors