chore: remove dead code - unused imports and variables

- Remove unused import OhMyOpenCodeConfig from src/index.ts
- Remove unused import dirname from src/features/opencode-skill-loader/loader.ts
- Remove unused import detectKeywords from src/hooks/keyword-detector/index.ts
- Remove unused import CliMatch from src/tools/ast-grep/utils.ts
- Prefix unused parameter _original in src/tools/ast-grep/utils.ts

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/features/opencode-skill-loader/loader.ts

@@ -1,5 +1,5 @@
 import { existsSync, readdirSync, readFileSync } from "fs"
-import { join, basename, dirname } from "path"
+import { join, basename } from "path"
 import { homedir } from "os"
 import yaml from "js-yaml"
 import { parseFrontmatter } from "../../shared/frontmatter"