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().