revert(plugins): drop UTF-8 BOM on SKILL.md — Codex YAML parser breaks on it

BOM before `---` makes Codex's strict frontmatter parser reject every
SKILL.md ("Skipped loading 66 skill(s)"). Without BOM the skills load
and execute correctly; the only remaining issue is mojibake display of
Cyrillic in SKILL.md previews — that's a Codex rendering bug, not ours.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/build-ports.yml

@@ -206,15 +206,6 @@ jobs:
           sed -e "s|{{PLUGIN_NAME}}|${PLUGIN_NAME}|g" \
             .github/templates/claude-plugin.json.tmpl > build/.claude-plugin/plugin.json
 
-      - name: Add UTF-8 BOM to SKILL.md (Codex reads Cyrillic as CP1252 without BOM)
-        if: matrix.platform == 'codex'
-        run: |
-          find build -name 'SKILL.md' -print0 | while IFS= read -r -d '' f; do
-            if ! head -c 3 "$f" | od -An -tx1 | tr -d ' \n' | grep -q '^efbbbf'; then
-              { printf '\xef\xbb\xbf'; cat "$f"; } > "$f.tmp" && mv "$f.tmp" "$f"
-            fi
-          done
-
       - name: Copy LICENSE
         run: cp LICENSE build/LICENSE