docs(contributing): align contributor docs with real workflow + fix ktlint command (#960)

- Add "search for dupes first" rule (open/closed PRs + issues) before starting.
- Fix ktlint instructions: the authoritative command is ./gradlew ktlintCheck
  / ktlintFormat, not the standalone ./ktlint binary (hand-fixes fail CI).
- Document rebase-onto-main before squash-merge (stale-branch overwrite trap).
- Add test-storage hygiene rule (no real on-device storage; TemporaryFolder /
  in-memory Room) — generic, not tied to any specific test harness or box.
- Add dependency-pinning note (AGP/Kotlin/SDK "SDK not writable" trap).
- Tighten PR template: type-of-change, rebased, ktlintCheck, navigateTo().

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,19 +1,35 @@
 ## Summary
 
-<!-- One sentence. -->
+<!-- One sentence: what + why. -->
 
 Fixes #
 
+## Type of Change
+
+- [ ] 🐛 Bug fix
+- [ ] ✨ Feature
+- [ ] ♻️ Refactor (no behavior change)
+- [ ] 📝 Docs
+- [ ] ✅ Tests
+- [ ] 🔧 CI / chore
+
 ## What changed
 
-<!-- What was wrong and what this PR does about it. -->
+<!-- What was wrong and what this PR does about it. Link the root cause if known. -->
 
 ## How to test
 
-<!-- Steps to verify. -->
+<!-- Steps to verify. For UI: which screen + which emulator/device API level. For WS: which RPC. -->
+
+1.
+2.
+3.
 
 ## Checklist
 
-- [ ] ktlint and CI pass
-- [ ] Tested on a device or CI APK
-- [ ] Navigation goes through `NavigationController.navigateTo()`
+- [ ] Branch rebased onto `main` (`git rebase origin/main`)
+- [ ] `./gradlew ktlintCheck` passes (ran `ktlintFormat` first)
+- [ ] `./gradlew testDebugUnitTest` green (or CI unit-tests job)
+- [ ] `checkColorLiterals` passes (no hardcoded Color outside theme/)
+- [ ] Navigation goes through `NavigationController.navigateTo()` (not `backStack.add`)
+- [ ] Tested on device/emulator or verified via CI APK