docs(skills): add beat-synced montage authoring recipe

[e-jung]

Closes #1623.

What

Adds a single new docs-only skill, skills/montage/SKILL.md, that closes the loop from the shipped beats CLI to a finished montage: given a folder of assets + beats/<audio>.json, place one segment per asset on a beat cadence, crossfade with the required gsap_exit_missing_hard_kill hard-kill, and duck #bgm under each clip that carries its own audio.

It is a docs-only change — one new file, no engine/runtime/CLI edits, no new commands. It builds entirely on the shipped beats CLI and the existing core media/volume contract (animate volume on the timeline; data-volume is the static baseline).

Why

As raised in #1623: hyperframes beats (#1424) and Studio beat-snapping (#1439) cover beat detection and manual snapping, but nothing closes the loop from a detected-beats file to a finished, beat-synced montage. This skill is the agent-facing recipe for that.

The recipe covers (per the issue)

1. Run hyperframes beats → read beats/<audio>.json ({ time, strength }[]).
2. Place one segment per asset on a beat cadence (every Nth beat), biasing cut points on strength.
3. Crossfade exits with the matching tl.set(..., { opacity: 0 }, boundary) hard kill required by the gsap_exit_missing_hard_kill lint rule.
4. Duck #bgm volume under each video clip and lift the clip's own <audio> (muted <video> + sibling <audio>).

Verification

• The worked example in the skill was extracted and run through npx hyperframes lint → 0 errors, 0 warnings (once the referenced assets/* placeholders exist; lint treats missing local media as errors).
• Conventions matched against existing single-file skills (general-video, slideshow): same frontmatter (name / description / metadata.tags), reads /hyperframes-core first, ends with a Validation section.
• oxfmt clean; conventional-commit message.

Scope notes

• Maintainer approval: @miguel-heygen approved opening this in Agent recipe: author a beat-synced montage from the beats CLI output? #1623 ("yeah feel free to open the pr").
• I did not wire this into the /hyperframes router cheat-sheet — keeping to a single docs file as scoped. Happy to add a one-line router entry as a follow-up if you want it discoverable there.
• AI disclosure: Human-reviewed — authored and lint-verified by the contributor behind the @e-jung account.

[e-jung]

Superseded by skills/music-to-video (merged) — it covers the beat-synced montage case comprehensively (and adopted the concept via references/montage.md). Closing in favor of the in-tree skill. Thanks!