File Checkpointing Enablement

What: Unlocked the file checkpointing ("Rewind") feature that allows reverting file changes to previous snapshots.

Details:

• Feature existed in previous versions but was hardcoded to disabled
• Now activatable via the tengu_use_file_checkpoints feature flag
• UI toggle added in settings: "Checkpointing (Rewind)" (line 400220)
• Can be disabled via CLAUDE_CODE_DISABLE_FILE_CHECKPOINTING environment variable
• Tracks file modifications and creates snapshots at message boundaries
• Enables rewinding files to previous states during conversations
• Disabled in headless mode
• Still behind a feature flag for gradual rollout
• Evidence: Activation function u7() at line 380874, UI controls at lines 400220-400237