feat: add clipboard API for remote browser clipboard access

POST /api/clipboard with { text } broadcasts to all connected browsers
via SSE. Browser attempts navigator.clipboard.writeText, falling back
to a modal with manual copy button if blocked.

Enables remote clipboard workflows: CLI tools can push text to the
user's browser clipboard across the network.

src/web/routes/index.ts

@@ -15,4 +15,5 @@ export { registerRespawnRoutes } from './respawn-routes.js';
 export { registerRalphRoutes } from './ralph-routes.js';
 export { registerPlanRoutes } from './plan-routes.js';
 export { registerOrchestratorRoutes } from './orchestrator-routes.js';
+export { registerClipboardRoutes } from './clipboard-routes.js';
 export { registerWsRoutes } from './ws-routes.js';