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/rpc.ts

@@ -0,0 +1,34 @@
+/**
+ * RPC singleton — breaks the circular import chain.
+ *
+ * main.ts creates the Electroview and RPC, then sets it here.
+ * All other modules import from this file instead of main.ts.
+ */
+
+import type { PtyRPCSchema } from '../shared/pty-rpc-schema.ts';
+
+// Placeholder type — matches the shape Electroview.defineRPC returns.
+// Uses `any` for the internal Electrobun RPC wrapper type since it is
+// not exported from the electrobun package.
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+type ElectrobunRpc = any;
+
+let _rpc: ElectrobunRpc | null = null;
+
+/** Called once from main.ts after Electroview.defineRPC(). */
+export function setAppRpc(rpc: ElectrobunRpc): void {
+  _rpc = rpc;
+}
+
+/**
+ * The app-wide RPC handle.
+ * Safe to call after main.ts has executed (Svelte components mount after).
+ */
+export const appRpc: ElectrobunRpc = new Proxy({} as ElectrobunRpc, {
+  get(_target, prop) {
+    if (!_rpc) {
+      throw new Error(`[rpc] accessed before init — property "${String(prop)}"`);
+    }
+    return (_rpc as Record<string | symbol, unknown>)[prop];
+  },
+});