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Add annotations as JSON column on tasks table with Annotate/Denotate methods and CLI commands. Add undo system backed by change_log with lightweight undo_stack table (capped at 10 entries). All mutating CLI commands (add, done, delete, modify, start, stop) now record undo entries. Undo restores prior task state from change_log data. Schema changes (in v1 migration): - annotations TEXT column on tasks - undo_stack table - annotations field in change_log triggers Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>