fix(P4): trim/split 8 overflowing slides + remove header (REQ-261)

Split slide 3 (Objectives + Anti-Goals) into Slide 3 (Objectives)
+ Slide 4 (Anti-Goals). Split slide 8 (Attestation Matrix) into
Slide 9 (QA concerns, 3 rows) + Slide 10 (Prod/DR concerns, 7 rows).
Main slide count 18 -> 20.

Trimmed: slide 7 (Pipeline) reduced to 3 bullets (4th covered by
diagram). slide 11 (Telemetry) reduced to 3 bullets. slide 14
(Deferred) merged 3 Live-AWS rows into 1 (8 -> 6 rows). slide 17
(Quarter-by-Quarter) dropped Grounding column (5 -> 4 cols). Global
table cell padding reduced (6px 10px -> 4px 8px) so 8-13 row tables
fit.

Removed header: from frontmatter (keep footer: + paginate only).
The full 51-char deck title in BOTH header and footer was redundant
chrome eating ~35px on every slide.

Updated test_marp_deck_slide_count (18 -> 20 main + 1 appendix).
Updated README slide-count convention (18 -> 20).

---ci---
project: acdl
phase: 4
milestone: v1.22
status: execute
phase_role: execution
---/ci---

.ciagent/CHECKPOINT.json

@@ -1,12 +1,12 @@
 {
-  "phase": 2,
+  "phase": 3,
   "stage": "complete",
   "milestone": "v1.22",
   "phase_role": "execution",
   "attempts": 0,
-  "updated_at": "2026-08-11T14:50:00Z",
+  "updated_at": "2026-08-11T14:55:00Z",
   "milestone_complete": false,
-  "tag": "v1.21.2",
-  "requirements": ["REQ-257","REQ-258"],
-  "notes": "v1.22 P2 complete. Tag v1.21.2. render_deck.sh deleted, CLI versions pinned, 2x scale + transparent bg added. 4 render tests + 1 no-forge test pass. Proceeding to P3 (mermaid re-layout)."
+  "tag": "v1.21.3",
+  "requirements": ["REQ-259","REQ-260"],
+  "notes": "v1.22 P3 complete. Tag v1.21.3. Both mermaid diagrams re-rendered at 2x transparent. telemetry-live-ops: TB, 0.63 aspect. platform-pipeline: TB 4-node, 0.49 aspect. Marp deck directives updated to h:480. Proceeding to P4 (deck content)."
 }