feat: add unified test runner and testing gate rule

Create v2/scripts/test-all.sh (vitest + cargo + optional E2E via --e2e).
Add npm scripts: test:all, test:all:e2e, test:cargo.
Add .claude/rules/20-testing-gate.md requiring full suite after major changes.

.claude/rules/20-testing-gate.md

@@ -0,0 +1,32 @@
+# Testing Gate (Post-Implementation)
+
+Run the full test suite after every major change before considering work complete.
+
+## What Counts as a Major Change
+
+- New feature or component
+- Refactoring that touches 3+ files
+- Store, adapter, or bridge modifications
+- Rust backend changes (commands, SQLite, sidecar)
+- Build or CI configuration changes
+
+## Required Command
+
+```bash
+cd v2 && npm run test:all
+```
+
+This runs vitest (frontend) + cargo test (backend). For changes touching E2E-relevant UI or interaction flows, also run:
+
+```bash
+cd v2 && npm run test:all:e2e
+```
+
+## Rules
+
+- Do NOT skip tests to save time. A broken test suite is a blocking issue.
+- If tests fail, fix them before moving on. Do not defer test fixes to a follow-up.
+- If a change breaks existing tests, that's signal — investigate whether the change or the test is wrong.
+- When adding new logic, add tests in the same commit (TDD preferred, see rule 06).
+- After fixing test failures, re-run the full suite to confirm no cascading breakage.
+- Report test results to the user: pass count, fail count, skip count.