fix(skills): address PR #2110 review feedback

- SSOT: the three assemble-index.mjs BGM fallbacks now import
  bgmDefaultVolume() from media-use's bgm.mjs instead of duplicating the
  0.12/0.9 literals (both reviewers). The cross-skill relative import
  matches the existing dependency (each workflow's audio.mjs adapter
  already resolves ../../media-use/audio/scripts/audio.mjs).
- STATUS_ROLE_KEY: extended with info|neutral|alert|caution|critical —
  same hue-carries-meaning class as the original set (all 3 copies).
- bgm.md: phrase the default as bgmDefaultVolume()/BGM_BED_VOLUME with
  "currently 0.12" so the prose survives future tuning.
- fetch-pr.mjs: drop dead mergeCommit field from the gh pr view FIELDS
  list (version resolution uses mergedAt only).
- music-to-video assemble-index.mjs: comment documenting why its BGM
  stays at 0.8 under VO — music is the content there, not a narration
  bed, so the explainer pipelines' 0.12 default deliberately does not
  apply.

Not changed: pickAccent's chroma fallback — both call sites pass keyless
capture palettes (tokens.json hex lists), so no status-role keys exist
to filter on; the keyed preset path goes through semanticColors and the
build-frame remix, which this PR already fixed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

skills/pr-to-video/scripts/fetch-pr.mjs

@@ -86,7 +86,6 @@ const FIELDS = [
   "mergedBy",
   "state",
   "mergedAt",
-  "mergeCommit",
 ].join(",");
 
 const view = ghTry(["pr", "view", prRef, "--json", FIELDS]);