Add beat-synced vlog editing skill

[DavidYu75]

Summary

Adds skills/beat-synced-vlog-editing: cutting vlogs and montages to music — lock a hero moment to the drop, cuts on the beat grid, musically motivated entrances, and keeping sync intact through later edits (the part that actually breaks).

What's inside

• Drop math on detect_beats output, including two traps: reported bpm can disagree with the beats array by ~2% (grid derives from median array spacing instead), and downbeats[] contains near-duplicates that need filtering.
• Entrance selection: music-from-frame-0 vs cold open as the first decision; mid-video entrances must start at the bottom of a build — riser before the chosen drop > another rising section > manufactured swell, with the RMS energy map (already computed for drop-finding) driving the choice.
• Three edit cases that break sync — before the music's start, inside its span, after the drop — with the recovery for each, including the whole-bar source retrim that absorbs a mid-span insert without tearing the entrance off its cut.
• Mix mechanics in dB with duck-recovery anchoring, and an explicit craft/taste boundary: universal defaults are encoded (duck under speech), style choices defer to the creator's brief with ask-don't-assume.
• Verification that catches what metadata can't: per-frame luma pass to locate single-frame hero moments, read-backs for floor-rounded trims/durations, and RMS analysis of the exported mix (a broken envelope passes every timeline check).

Validation

Four rounds of cold tests: fresh agents (two model tiers) given only this SKILL.md, MCP access, and footage/songs they'd never seen. Each round's failure report became the next revision; the rules here all trace to something that actually went wrong. The demo below is round 4's unedited output — lights-on lands on the drop frame-exact, and the entrance rides the song's own verse build.

demo_beatsync_test.MP4

Palmier behaviors found while testing (documented as workarounds in the skill)

1. set_keyframes volume values render as dB, but the tool description says 0.0–1.0 linear (set_clip_properties volume IS linear). A docs-conformant envelope renders flat — no fades, no ducks — and passes every non-audio check. Happy to file as an issue with the probe methodology.
2. insert_clips landing inside a placed music clip's span splits it and leaves a silent gap; the inserted clip's linked audio can land on the music's track. Step 3 documents the rebuild.
3. Source-seconds→frames conversions floor throughout: 60fps sources on a 30fps timeline routinely come back one frame short, and trims can floor half a frame early.

catalog.json regenerated with scripts/build-catalog.mjs. Happy to iterate on scope or structure. Natural follow-up skill: sound design / SFX layering.