docs(01): revise plan 07 wave + ffprobe verify guard per checker iteration 1

Two changes:
1. wave: 3 → 6 (cascade: max(wave(05)=4, wave(06)=5)+1 = 6).
2. Task 1 <automated> verify now prefixes the ffprobe invocation with
   test -f tests/fixtures/last_30sec.webm && which ffprobe so the gate
   fails fast with a clear signal if the human checkpoint never produced
   the fixture (instead of ffprobe blowing up with a cryptic file-not-found).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.planning/phases/01-stabilize-video-pipeline/01-07-PLAN.md

@@ -2,7 +2,7 @@
 phase: 01-stabilize-video-pipeline
 plan: 07
 type: execute
-wave: 3
+wave: 6
 depends_on: ["05", "06"]
 files_modified:
   - tests/fixtures/last_30sec.webm
@@ -139,7 +139,7 @@ The full Phase-1 Mokosh build:
 - Archive is delivered via `chrome.downloads`
   </what-built>
   <verify>
-    <automated>which ffprobe && ffprobe -v error -f matroska -i tests/fixtures/last_30sec.webm; test $? -eq 0</automated>
+    <automated>test -f tests/fixtures/last_30sec.webm && which ffprobe && ffprobe -v error -f matroska -i tests/fixtures/last_30sec.webm; test $? -eq 0</automated>
   </verify>
   <how-to-verify>
 **Pre-flight (automated, do these in a bash shell before opening Chrome):**