docs: add monorepo management rules (#4296)

Co-authored-by: Eric Wheeler <[email protected]>

Author: KJ7LNW

.roo/rules/monorepo.md

@@ -0,0 +1,51 @@
+# Monorepo (pnpm)
+
+- Work primarily in:  
+  • src/  
+  • webview-ui/package  
+  • packages/types (shared DTOs)
+- Other workspaces (modify only if instructed):  
+  • evals/  
+  • packages/_/  
+  • apps/_/  
+  • [others]
+- To install deps:  
+  → run `pnpm add <package>` in `<cwd>workspace/directory</cwd>` (updates `<workspace>/package.json`)
+- ⚠️ DO NOT run pnpm in root (root only holds CLI dev-tools in devDependencies, e.g. prettier)
+
+# Turborepo
+
+- ❌ Don’t modify `turbo` config without instruction
+- ✅ You may propose config changes when:
+    1. Adding a shared, cacheable npm task
+    2. Declaring an internal-package dependency that affects the extension build
+
+# Running tests
+
+- Always execute commands using `<cwd>workspace/directory</cwd>` when running any test command
+- To fix failures:
+    1. Target one failing test:  
+       → `npx vitest relative/path/__tests__/name.spec.ts -t "substring"`
+    2. Verify all tests in that module pass:  
+       → `npx vitest relative/path/__tests__/name.spec.ts`
+- File conventions:  
+  • `.spec.ts` → vitest  
+  • `.test.ts` → jest (deprecated)
+- If a test in `.test.ts` fails:  
+  → delete it & recreate in `.spec.ts`
+- Keep passing tests in `.test.ts`
+- Use `npx jest <file> -t "substring"` for deprecated test execution using the same strategy above
+- Run all tests in the root workspace:  
+  → `pnpm test`
+
+# Creating tests
+
+- Use vitest (native ESM, jest-compatible)
+- Steps:
+    1. Create `path/to/__tests__/name.spec.ts` (must be `.spec.ts`)
+    2. Omit ESM mocks unless necessary
+    3. Run via vitest
+- For modules with existing `.test.ts`:
+    1. Move any failing tests → `name.spec.ts`
+    2. Add new tests → `name.spec.ts`
+    3. Leave successful tests in `.test.ts`