docs(P2): restore clean style — theme:default + inline style (REQ-265,266,267)

Reverted frontmatter theme: nova-sp -> theme: default + inline style:
block with S&P palette (#D6002A, #1B1B1B, Akkurat Pro). Retired
nova-sp-theme.css from render path (kept as reference with header
comment). render_slides.sh drops --theme arg. Converted all 21
**Benefit:** callouts to <div class='benefit'> (red top-rule + black
italic; white on title slides). Matches the old
the-developer-experience.html clean style.

---ci---
project: acdl
phase: 2
milestone: v1.23
status: execute
phase_role: execution
---/ci---

docs/presentations/assets/nova-sp-theme.css

@@ -1,3 +1,8 @@
+/* RETAINED AS REFERENCE ONLY — not loaded at render time.
+ * The live deck uses Marp `default` theme + an inline `style:` block in
+ * the -marp.md frontmatter. This file is kept for future styling work
+ * reference. Do NOT pass via `--theme`; it is not in the render path.
+ */
 /* @theme nova-sp */
 /* Nova — S&P Global Energy theme for Marp decks.
  *