fix: protect job frontmatter from being overwritten during execution

[Aleschka]

Summary

• Adds snapshotJobFrontmatter() to jobs.ts — takes a snapshot of the job file's YAML frontmatter before execution and returns a restore function
• Integrates snapshot/restore into the job execution loop in start.ts — if Claude modifies or strips the frontmatter during a run, it is automatically restored
• Handles edge cases: file deleted, frontmatter completely mangled, or body content changed by Claude (preserves updated body, restores only frontmatter)
• Logs a message when restoration occurs for debugging

Fixes #1

Test plan

• Create a job with schedule and recurring frontmatter
• Run the job and verify frontmatter remains intact
• Manually strip frontmatter from a job file mid-run to simulate the bug, verify it gets restored
• Verify one-shot jobs still clear their schedule correctly after execution