style: apply Prettier formatting to TS/JS/JSON/MD files

Add Prettier as a dev dependency with .prettierrc.json config and
.prettierignore. Hook added via mirrors-prettier in pre-commit config.
All TypeScript, JSON, Markdown, and YAML files auto-formatted.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.prettierrc.json

@@ -0,0 +1,7 @@
+{
+  "semi": true,
+  "trailingComma": "all",
+  "singleQuote": false,
+  "printWidth": 100,
+  "tabWidth": 2
+}