refactor(taskctl): extract generateDiffForRespawn utility from respawnDeveloper

[randomm]

Task Description

The respawnDeveloper function in pulse-scheduler.ts has grown to ~200 lines and exceeds the single responsibility principle. Extract the git diff generation logic into a dedicated utility function.

Quality Gates

• TDD: Write tests before implementation
• Coverage: 80%+ test coverage for new utility
• Linting: All code passes project linting rules
• Documentation: Update README.md if needed

Acceptance Criteria

• Create generateDiffForRespawn(worktree: string, baseCommit: string | null, taskId: string): Promise<string> utility function
• Move git diff generation logic (ancestor check, diff generation, size limits, worktree validity) from respawnDeveloper to the new utility
• respawnDeveloper should be a focused orchestrator function (~100 lines)
• All existing tests still pass
• Add unit tests for the new utility function