build: adopt esbuild for frontend bundling (build:web → public/build/)

- esbuild dev dep; build:web/dev:web bundle public/main.ts → public/build/main.js
- gitignore public/build/; tsconfig.web.json now typecheck-only
- frontend build convention documented in PLAN §1, DISPATCH, ARCHITECTURE §5
- resolves the T1 deferred decision

.gitignore

@@ -3,8 +3,7 @@ node_modules/
 
 # build output
 dist/
-public/*.js
-public/*.js.map
+public/build/
 
 # local Claude Code settings (not shared)
 .claude/settings.local.json