feat: implement iterate run + iterate watch (file-based persistence, agent invocation)

[emil07770]

Summary

iterate run and iterate watch are currently stubs. This issue tracks their full implementation.

What’s needed

iterate run

• Load goals from ~/.sh1pt/iterate-goals.json
• Determine metric signals from --scope (copy | pricing | onboarding | perf | bugs | all)
• Capture/compare a MetricSnapshot stored in ~/.sh1pt/iterate-metrics.json
• Build a structured agent prompt from goals + signals + scope
• Invoke the configured agent (claude, codex, or qwen) via CLI with --print
• Save a RunRecord (id, timestamps, agent, scope, goals, status, diff) to ~/.sh1pt/iterate-runs.json (capped at 100 entries, atomic writes)
• --dry-run flag shows plan + prompt without invoking agent
• --auto-apply skips confirmation prompt
• --json emits machine-readable run record

iterate watch

• Save/load WatchConfig to ~/.sh1pt/iterate-watch.json (atomic writes)
• --status shows current config
• --stop removes the config file
• --cloud prints deployment instructions for sh1pt cloud
• Local mode prints a ready-made crontab line using --interval and --agent
• --quiet-hours <start-end> (e.g. 22-08) skips cycles during off-hours

Persistence pattern

All files use atomic write (write .tmp → rename) with mode: 0o600, same as existing goals implementation.

Acceptance criteria

• sh1pt iterate run --dry-run shows plan + prompt, saves skipped RunRecord
• sh1pt iterate run --agent claude invokes claude --print <prompt>, saves applied RunRecord
• sh1pt iterate run --auto-apply skips confirmation
• sh1pt iterate run --json emits JSON run record
• sh1pt iterate watch saves WatchConfig, prints crontab line
• sh1pt iterate watch --cloud prints cloud deployment info
• sh1pt iterate watch --stop clears config
• sh1pt iterate watch --status shows current config + quiet-hours state
• All state files use atomic writes, mode 0o600