Add rollback support for Claude agent SDK file edits

[Copilot]

The Claude agent SDK supports reverting file edits to a previous conversation state via rewindFiles(), but this wasn't exposed in the VS Code integration.

Changes

• Enabled file checkpointing in Claude session options (enableFileCheckpointing: true)
• Exposed rewindFiles() methods on both ClaudeCodeSession and ClaudeAgentManager
  • Session-level: rewindFiles(userMessageId, dryRun?)
  • Manager-level: rewindFiles(sessionId, userMessageId, dryRun?)
• Added test coverage for rewind operations, error cases, and dry-run mode
• Updated documentation with API details and use cases

Usage

// Revert to a specific message in the conversation
const result = await claudeAgentManager.rewindFiles(
    sessionId,
    userMessageId,
    false // set true for dry-run preview
);

// Returns: { canRewind: boolean, filesChanged?: string[], insertions?: number, deletions?: number, error?: string }

Each user message in a Claude session now serves as a potential restore point. The SDK automatically maintains file backups when checkpointing is enabled.

Note: This provides the API foundation. UI integration (commands, buttons, dialogs) is tracked separately as future work.

Original prompt

The Claude agent SDK has a concept of rolling back edits to a particular point in time. Implement handling of this in the claudeCodeAgent so that I can revert edits with ease

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