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/music-to-video/scripts/assemble-index.mjs

@@ -153,7 +153,9 @@ for (const m of mounted) {
   body.push("");
 }
 
-// BGM (track 11) — full duration; duck slightly when VO present
+// BGM (track 11) — full duration. Here the music IS the content (music-first
+// skill), so it never drops to the explainer pipelines' narration-bed default
+// (bgmDefaultVolume() 0.12 ≈ -18 dB): an incidental VO ducks it only slightly.
 let bgmEmitted = false;
 if (existsSync(join(hyperframesDir, bgmRel))) {
   const vol = voiceCount > 0 ? 0.8 : 0.9;