early exit if not files created/changed handlePostTask

[gtrrz-victor]

Note

Ensures PostTask does not create a checkpoint when a task results in no file changes, and still cleans up pre-task state.

• In handlePostTask, early-return when FilterAndNormalizePaths yields no modified or new files; log and best-effort CleanupPreTaskState
• Add integration test TestSubagentCheckpoints_PostTaskNoFileChanges to verify no commits are created and pre-task file is removed
• Maintains existing behavior for PostTodo incremental checkpoints (no changes there)

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[Copilot]

Pull request overview

This PR adds an early exit optimization to the handlePostTask function to skip creating a checkpoint when no file changes are detected (neither modified nor newly created files).

Changes:

• Adds an early exit condition in handlePostTask that checks if both relModifiedFiles and relNewFiles are empty
• Includes cleanup of pre-task state before returning when no changes are detected
• Outputs a user-friendly message indicating the checkpoint is being skipped