fix(P2): render scripts — delete render_deck.sh, pin versions, 2x scale (REQ-257,258)

REQ-257: deleted scripts/render_deck.sh (omitted --theme, produced
unthemed output; README already documents render_slides.sh as
canonical). Pinned marp-cli@4.5.0 + mermaid-cli@11.16.0 in
render_slides.sh to prevent boilerplate-CSS drift. Removed
render_deck.sh references from README, sync_to_nova.sh, and
test_no_forge_mentions.py.
REQ-258: added -s 2 -b transparent to mermaid-cli invocation (matches
README spec line 193). Produces crisp 2x PNGs with transparent
backgrounds instead of 1x renders.

---ci---
project: acdl
phase: 2
milestone: v1.22
status: execute
phase_role: execution
---/ci---

tests/test_no_forge_mentions.py

@@ -40,7 +40,7 @@ _EXCLUDE_SCRIPTS = {
     "untag_acdl_keys.py", "seed_uptime_monitors.py",
     "push_consumer_image.py", "sync_workflows.py",
     "attach_release_asset.py", "check_north_star_diff.sh",
-    "render_deck.sh", "render_slides.sh",
+    "render_slides.sh",
 }
 
 # Synced top-level files (not in any excluded dir).