docs: fix Phase 20 test count — 48 not 49

Verified by counting: T1(5) + T2(12) + T3(8) + T4(8) + T5(13) + Proto(2) = 48.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sw-block/.private/phase/phase-20-test.md

@@ -183,7 +183,7 @@ Legend:
 
 ## Phase 20 Current Coverage Reading
 
-Post-Tier-1 component test implementation. 49 total tests.
+Post-Tier-1 component test implementation. 48 total tests.
 
 | Capability | Unit | Component | Integration | Hardware / Runner | Current reading |
 |---|---|---|---|---|---|
@@ -922,6 +922,10 @@ This should currently be read as:
 
 They should execute in stages.
 
+Execution details, scenario packs, and observation checklists live in:
+
+1. `sw-block/.private/phase/phase-20-t6-runbook.md`
+
 ### Stage 0: Bootstrap Closure Prerequisite
 
 Goal: