docs: rename test_sprint21 to test_sprint20b, update test counts to 415

Sprint 20 combines voice input (20a) and send button polish (20b).

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

CHANGELOG.md

@@ -6,7 +6,7 @@
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 ## [v0.22] Sprint 20 -- Voice Input + Send Button Polish
-*April 3, 2026 | 382 tests*
+*April 3, 2026 | 415 tests*
 
 ### Features
 - **Voice input via Web Speech API.** Microphone button in the composer.
@@ -39,7 +39,9 @@
 
 ### Tests
 - 52 new tests in `test_sprint20.py`: voice input HTML, CSS, JS, append
-  behaviour, error handling, regressions. Total: **382 tests**.
+  behaviour, error handling, regressions.
+- 33 new tests in `test_sprint20b.py`: send button HTML, CSS, JS,
+  animation, visibility logic, regressions. Total: **415 tests**.
 
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@@ -714,4 +716,4 @@ Three-panel layout: sessions sidebar, chat area, workspace panel.
 
 ---
 
-*Last updated: v0.22, April 3, 2026 | Tests: 382*
+*Last updated: v0.22, April 3, 2026 | Tests: 415*