feat: auto-persist WAA recordings to prevent data loss

[abrichr]

Summary

• Add waa_recordings/ to .gitignore — recordings are now immune to git stash -u and git clean -f
• Add _backup_file() helper: hardlinks PNGs + meta.json to ~/oa/recordings/{task_id}/ (zero extra disk space, falls back to copy on cross-device, silent on failure)
• Add _save_incremental_meta(): writes meta.json atomically (via .tmp rename) after each step with recording_complete field for partial recording detection
• Wire helpers into the recording loop (before/after screenshots, step advances, done, restart cleanup)
• Use systemd-first pattern for socat proxy in auto-infrastructure

Protection matrix

Threat	Protected?	Mechanism
git stash -u	YES	.gitignore
git clean -f	YES	.gitignore
git clean -fx	YES	External backup
Script crash mid-recording	YES	Incremental meta.json
Accidental rm -rf repo/	YES	External backup

Test plan

• Run python scripts/record_waa_demos.py record-waa — verify meta.json appears after first step, not just at end
• Verify ~/oa/recordings/{task_id}/ has hardlinked copies of PNGs
• Run git stash -u in repo — verify waa_recordings/ is NOT removed
• Verify convert_recording_to_demo.py still parses the new meta.json with recording_complete field

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