docs(project-guide): require yarn tsc --noEmit before frontend commits

Vite's HMR dev server is permissive — it doesn't fail on unused
vars/imports, but Cloud Build's tsc strict mode does (TS6133). Add an
explicit pre-commit step + common traps (unused prop on XxxProps after
removing its use, leftover import after dropping a feature). Saves a
Cloud Build round-trip.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: MarkusNeusinger

agentic/docs/project-guide.md

@@ -94,10 +94,21 @@ uv run ruff check <files> && uv run ruff format <files>
 ```bash
 cd app
 yarn install
-yarn dev          # Development server
-yarn build        # Production build
+yarn dev               # Development server
+yarn tsc --noEmit      # Type-check only (catches TS6133 unused vars etc.)
+yarn build             # Production build (runs tsc + vite build)
 ```
 
+**IMPORTANT: Run `yarn tsc --noEmit` (or `yarn build`) before committing frontend changes.**
+Vite's HMR dev server is permissive — it does NOT fail on unused variables, unused imports,
+or other TS strict errors. Cloud Build runs `tsc && vite build` and will fail on any TS6133
+("declared but never read") errors. Catching these locally before `git push` saves a Cloud
+Build round-trip. Common traps:
+
+- Removing a prop's usage from a component body but forgetting to remove it from `XxxProps`
+- Removing a feature (e.g. color accents) but leaving the import (`import { colors }`)
+- Changing a hook's shape and leaving a now-unused destructured name
+
 ## Architecture
 
 ### Plot-Centric Design