Format all project startup docs with Prettier

- Add Prettier as dev dependency
- Apply formatting to interview map, retrospective, and planning document

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

package.json

@@ -0,0 +1,5 @@
+{
+  "devDependencies": {
+    "prettier": "^3.8.2"
+  }
+}