docs: fix test count (294→318) and add Sprint 17 Tests section

CHANGELOG.md and SPRINTS.md incorrectly stated 294 tests for v0.19.
Actual count is 318 (289 from v0.18.1 + 6 new Sprint 17 + 23 in
test_regressions.py). The Sprint 17 commit message miscounted as
'5 new' when test_sprint17.py contains 6 tests.

Also adds a Tests section to the Sprint 17 CHANGELOG entry listing
what the 6 tests cover, and notes the send_key enum validation.

Tests: 318 passed, 0 failed.

CHANGELOG.md

@@ -6,7 +6,7 @@
 ---
 
 ## [v0.19] Sprint 17 -- Workspace Polish + Slash Commands + Settings
-*April 3, 2026 | 294 tests*
+*April 3, 2026 | 318 tests*
 
 ### Features
 - **Workspace breadcrumb navigation.** Clicking into subdirectories now shows a
@@ -28,9 +28,15 @@
 ### Architecture
 - New `static/commands.js` module (7th JS module): command registry, parser,
   autocomplete dropdown, and built-in command handlers.
-- `send_key` added to `_SETTINGS_DEFAULTS` in `api/config.py` with enum validation.
+- `send_key` added to `_SETTINGS_DEFAULTS` in `api/config.py` with enum validation
+  (`_SETTINGS_ENUM_VALUES` rejects unknown values server-side).
 - `S.currentDir` state tracking added to `ui.js` for workspace navigation.
 
+### Tests
+- 6 new tests in `test_sprint17.py`: send_key default, round-trip save with
+  cleanup, invalid value rejection, unknown key ignored, commands.js served,
+  workspace root listing. Total: **318 passed**.
+
 ---
 
 ## [v0.18.1] Safe HTML Rendering + Sprint 16 Tests