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/sse-events.ts

@@ -319,6 +319,11 @@ export const OrchestratorCompleted = 'orchestrator:completed' as const;
 /** Orchestrator error. */
 export const OrchestratorError = 'orchestrator:error' as const;
 
+// ─── Clipboard ──────────────────────────────────────────────────────────────
+
+/** Clipboard content pushed to browser. */
+export const ClipboardWrite = 'clipboard:write' as const;
+
 // ─── Cases ───────────────────────────────────────────────────────────────────
 
 /** New case directory created. */
@@ -486,6 +491,9 @@ export const SseEvent = {
   OrchestratorCompleted,
   OrchestratorError,
 
+  // Clipboard
+  ClipboardWrite,
+
   // Cases
   CaseCreated,
   CaseLinked,