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/SecuritySettings.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
   import { onMount } from 'svelte';
-  import { appRpc } from '../main.ts';
+  import { appRpc } from '../rpc.ts';
 
   const KNOWN_KEYS: Record<string, string> = {
     ANTHROPIC_API_KEY:  'Anthropic API Key',
@@ -77,6 +77,11 @@
 <!-- svelte-ignore a11y_no_static_element_interactions -->
 <div class="section" onclick={handleOutsideClick} onkeydown={e => e.key === 'Escape' && (keyDropOpen = false)}>
 
+  <div class="prototype-notice">
+    Prototype — secrets are stored locally in plain SQLite, not in the system keyring.
+    Do not store production credentials here.
+  </div>
+
   <h3 class="sh">Keyring Status</h3>
   <div class="keyring-status" class:ok={keyringAvailable} class:unavail={!keyringAvailable}>
     <span class="ks-dot"></span>
@@ -148,6 +153,15 @@
 </div>
 
 <style>
+  .prototype-notice {
+    padding: 0.5rem 0.625rem;
+    background: color-mix(in srgb, var(--ctp-peach) 12%, transparent);
+    border: 1px solid color-mix(in srgb, var(--ctp-peach) 30%, transparent);
+    border-radius: 0.25rem;
+    color: var(--ctp-peach);
+    font-size: 0.75rem;
+    line-height: 1.4;
+  }
   .section { display: flex; flex-direction: column; gap: 0.5rem; }
   .sh { margin: 0.375rem 0 0.125rem; font-size: 0.6875rem; font-weight: 700; text-transform: uppercase; letter-spacing: 0.06em; color: var(--ctp-overlay0); }