feat(skills): add beat-sync-montage workflow

[Alphaxiaoteng]

Adds a bundled skill for the case the repo's music tooling does not cover: the creative side of a beat-synced montage — what to cut and why, not just the mechanics of landing a cut on a beat.

Why it is not covered today

The music stack (music-intelligence, music) is tooling: analyze_music, music_edit_plan, sync_cuts_to_music, music_image_plan. It tells the agent how to sync cuts to music. It never says which shots belong on which beat, how dense to cut, or where to breathe. So a perfectly on-beat edit can still feel mechanical, tiring, or unrelated to the picture — and nothing in the library addresses that.

This skill fills the creative layer and hands off execution to music-intelligence (it does not re-explain the tools).

What the skill adds

SKILL.md — a 10-step workflow whose spine is decide density before picking cuts:

• Explicit When to switch workflows block routing photo/locked-track edits to music-intelligence (music_image_plan), long single-source speech to long-video-to-shorts, and non-music-driven highlight reels to multi-clips-to-reels
• Read the track first (BPM, meter, sections, energy shape) — never request the CLAP embedding
• Fitness check before planning: unsuitable footage is reported, not force-cut on every beat
• Density as a property of the whole piece (verse ≠ chorus), anchors-first planning, deliberate breathing room
• Motion-continuity QA (screen direction, axis, scale, luminance) that only shows up when watching, not when reading a cut list

references/beat-density.md — section map → density arc, the four levels (rest / sparse / medium / dense), anchors-first, breathing room, when to hold across or cut off the grid, and a capacity check capping density by usable onset shots.

references/shot-fitness.md — onset scoring, why fast BPM is not a licence to go dense, motion continuity across cuts, and the red flags (regular cut list, repeated shots, no visible onset) that mark a generated edit.

Test surface

Both roster checks assert exact counts, so they move with the addition:

• plugin-skills.verify.ts: beat-sync-montage added to EXPECTED; stale 26-skill comments corrected to 28
• skill-loading.verify.ts: bundled count 27 → 28

npm run verify:skills covers these. Locally the roster check passes:

plugin-skills.check: ok (28 技能 / EXPECTED=28)

skill-loading.verify.ts needs Vite and Node 24 (only Node 22 is available here), so it could not be executed in this environment — the one assertion it adds is the mechanical count bump above, plus the body-link check, which passes by inspection since both references/*.md files exist and are globbed into skill.files.

Notes for review: this is an independent, self-contained skill (no dependency on #141). If you would rather fold the creative guidance into music-intelligence as a reference instead of a new skill, say so and I will restructure — though music-intelligence is framed around tool usage throughout and has no workflow section today.