feat(P2): dedicated S&P theme + render pipeline + CI workflow (REQ-239..243)

Create nova-sp-theme.css — S&P Global Energy Marp theme (Red/Black/White
palette applied to all slide chrome: backgrounds, headers/footers, pagination,
tables, blockquotes, code blocks).
Create render_slides.sh — end-to-end pipeline: mermaid PNGs + Marp HTML/PPTX.
Create slides.yml CI workflow — auto-renders on docs/presentations/ changes.
Create test_slides_pipeline.py — 12 tests (theme CSS, Marp frontmatter, script,
workflow, .mmd/.png parity, README retired-deck cleanup).
Update Marp frontmatter: theme: nova-sp + footer v1.20.
Fix presentations/README.md directory layout (remove retired decks).
Re-render HTML + PPTX with S&P theme.

---ci---
project: acdl
phase: 2
milestone: v1.20
status: execute
requirements: [REQ-239, REQ-240, REQ-241, REQ-242, REQ-243]
---/ci---

scripts/sync_to_nova.sh

@@ -118,6 +118,7 @@ EXCLUDE_SCRIPTS=(
   attach_release_asset.py
   check_north_star_diff.sh
   render_deck.sh
+  render_slides.sh
 )
 
 CONV_RE='^(feat|fix|docs|chore|refactor|perf|test|build|ci|style|revert)(\([^)]+\))?: .+'