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/bun/message-adapter.ts

@@ -369,9 +369,11 @@ function adaptCodexItem(
         },
       ];
     case "command_execution": {
+      // Fix #13: Only emit tool_call on item.started, tool_result on item.completed
+      // Prevents duplicate tool_call messages.
       const messages: AgentMessage[] = [];
       const toolUseId = str(item.id, uuid);
-      if (eventType === "item.started" || eventType === "item.completed") {
+      if (eventType === "item.started") {
         messages.push({
           id: `${uuid}-call`,
           type: "tool_call",