feat: dev plugin install flow + smoke-platform

- .gitignore: add plugins/ (local dev install, never committed)
- scripts/install-dev-plugins.sh: install dist package from ../verstak-official-plugins/dist/ into ./plugins/
- scripts/smoke-platform.sh: headless verification of plugin discovery, manifest, capabilities, contributions
- cmd/smoke-platform/main.go: Go smoke command for headless plugin verification
- docs/DEV_PLUGINS.md: dev plugin flow documentation

.gitignore

@@ -1,3 +1,4 @@
 frontend/node_modules/
 frontend/dist/
 build/bin/verstak-desktop
+plugins/