fix(electrobun): address all 20 Codex review findings

CRITICAL:
- PTY leak: Terminal.svelte now calls pty.close on destroy, not just unsubscribe
- Agent session cleanup: clearSession() removes done/error sessions, backend
  deletes after 60s grace period

HIGH:
- Clone branch passthrough: user's branch name flows through callback
- Circular imports: extracted rpc.ts singleton, broke main.ts ↔ App.svelte cycle
- Settings wired to runtime: Terminal reads cursor/scrollback from settings
- Security disclaimer: added "prototype — not system keyring" notice
- ThemeEditor: fixed basePalette → initialPalette reference

MEDIUM:
- Clone race: UUID suffix instead of count-based index
- Silent failures: structured error returns from PTY handlers
- WebKitGTK mount: only current + previous group mounted
- Debug listeners: gated behind DEBUG, cleanup on destroy
- NDJSON residual buffer parsed on process exit
- Codex adapter: deduplicated tool_call/tool_result
- extraEnv: rejects CLAUDE*/CODEX*/OLLAMA* keys
- settings-db: runMigrations() with version tracking
- active_group: persisted via settings.set

LOW:
- Removed dead demo code, unused variables
- color-mix() fallbacks added

ui-electrobun/src/mainview/main.ts

@@ -4,6 +4,7 @@ import App from "./App.svelte";
 import { mount } from "svelte";
 import { Electroview } from "electrobun/view";
 import type { PtyRPCSchema } from "../shared/pty-rpc-schema.ts";
+import { setAppRpc } from "./rpc.ts";
 
 /**
  * Set up Electroview RPC.
@@ -31,9 +32,12 @@ const rpc = Electroview.defineRPC<PtyRPCSchema>({
   },
 });
 
+// Register the RPC singleton so all modules can import from rpc.ts
+setAppRpc(rpc);
+
 export const electrobun = new Electroview({ rpc });
 
-/** Exported for use by stores that need RPC access (theme-store, font-store). */
+/** @deprecated Import from './rpc.ts' instead. */
 export { rpc as appRpc };
 
 const app = mount(App, {