Add optional `metadata` field to tasks and subtasks for storing
arbitrary user-defined JSON data (external IDs, workflow data,
integration references, etc.).
Key features:
- AI-Safe: Metadata excluded from AI schemas, preserved through all operations
- MCP Support: update_task/update_subtask tools accept metadata parameter
- Safety Flag: MCP updates require TASK_MASTER_ALLOW_METADATA_UPDATES=true
- Metadata Merge: New metadata merges with existing, preserving unmodified fields
Closes#1555🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
