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/settings/AppearanceSettings.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
   import { onMount } from 'svelte';
-  import { appRpc } from '../main.ts';
+  import { appRpc } from '../rpc.ts';
   import { THEMES, THEME_GROUPS, getPalette, type ThemeId, type ThemeMeta } from '../themes.ts';
   import { themeStore } from '../theme-store.svelte.ts';
   import { fontStore } from '../font-store.svelte.ts';