chore: update gitignore and mark check command task complete

Author: kevinmichaelchen

.gitignore

@@ -7,6 +7,7 @@ Cargo.lock
 .helix/helix.db/
 .helix/models/
 .helix-map/
+.fastembed_cache/
 
 # IDE
 .idea/

helix-decisions/specs/tasks.md

@@ -77,7 +77,7 @@
 - [x] Output JSON format
 - [x] Auto-discovery of `.decisions/` directory
 - [x] Help text and examples
-- [ ] Add `check` command for frontmatter + uuid validation
+- [x] Add `check` command for frontmatter + uuid validation
 
 ### Task 1.8: Git Hooks ✅
 - [x] Implement `hooks.rs` module