fix(codex): strip alt-screen + scrollback-erase from the codex byte stream

Codex's TUI emits alternate-screen toggles (DECSET/DECRST 47/1047/1049),
scrollback-erase (CSI 3 J), and mouse-tracking enables (?1000-1007) during
startup and on every repaint. xterm.js obeys them: it switches to the
scrollback-less alternate buffer, wipes saved lines, and forwards the scroll
wheel to codex — so the user's conversation history both disappears and
becomes unreachable on each tab switch / pane refresh.

Strip these sequences in two places, leaving the visible-viewport erases
(2J / J) intact so codex can still repaint its own rows:

- Session._handleTerminalOutput: filter the live SSE/WS stream and the
  persisted terminal buffer at the source, for mode === 'codex'.
- GET /api/sessions/:id/terminal: apply the same strip to the replayed
  buffer (ALT_SCREEN_TOGGLE_PATTERN / ERASE_SCROLLBACK_PATTERN) so a
  tab-switch replay keeps full scrollback.

Adds test/codex-terminal-output.test.ts covering the strip (alt-screen and
3J removed, 2J/J preserved, Ctrl+L redraws preserved) and confirming codex
output passes through without Ink row-repair mangling.

Co-Authored-By: Saqeb Akhter <saqeb.akhter@gmail.com>

src/session.ts

@@ -1052,6 +1052,32 @@ export class Session extends EventEmitter {
   }
 
   private _handleTerminalOutput(data: string): void {
+    // Codex emits sequences that wipe xterm.js scrollback, plus mouse-tracking
+    // enables that hijack the scroll wheel so the user can't reach scrollback:
+    //   - \x1b[?1049h / \x1b[?47h / \x1b[?1047h: switch to the alt buffer (no
+    //     scrollback) — \x1b[?...l switches back.
+    //   - \x1b[3J: erase saved lines (scrollback). (\x1b[2J / \x1b[J — erase
+    //     the visible viewport — are left intact; the TUI repaints those rows.)
+    //   - \x1b[?1000h / 1002h / 1003h / 1005h / 1006h / 1007h: mouse-tracking
+    //     modes (X10, button-event, any-event, UTF-8, SGR, alt-scroll). Once on,
+    //     xterm.js forwards wheel events to codex instead of scrolling the
+    //     viewport, so the conversation is in scrollback but unreachable.
+    //     (Focus events at ?1004 are left alone — codeman uses them for
+    //     active-tab detection.)
+    // Strip them at the source so neither the persisted buffer nor the live
+    // SSE/WS stream carries them, keeping everything in the main buffer with
+    // scrollback intact. Codex's cursor-positioned redraws overwrite only the
+    // cells they actually target, so the non-erased rows keep their content.
+    if (this.mode === 'codex') {
+      data = data
+        // eslint-disable-next-line no-control-regex
+        .replace(/\x1b\[\?(?:47|1047|1049)[hl]/g, '')
+        // eslint-disable-next-line no-control-regex
+        .replace(/\x1b\[3J/g, '')
+        // eslint-disable-next-line no-control-regex
+        .replace(/\x1b\[\?(?:1000|1001|1002|1003|1005|1006|1007)[hl]/g, '');
+    }
+
     // BufferAccumulator handles auto-trimming when max size exceeded
     this._terminalBuffer.append(data);
     this._lastActivityAt = Date.now();

src/web/routes/session-routes.ts

@@ -65,6 +65,25 @@ const CLAUDE_BANNER_PATTERN = /\x1b\[1mClaud/;
 const CTRL_L_PATTERN = /\x0c/g;
 const LEADING_WHITESPACE_PATTERN = /^[\s\r\n]+/;
 
+/**
+ * Match xterm alternate-screen mode toggles + the standalone scrollback-erase.
+ *
+ * - DECSET/DECRST 47, 1047, 1049 = enter/exit alternate screen buffer
+ *   (1049 also saves cursor and clears the alt buffer).
+ * - CSI 3 J = erase saved lines (scrollback).
+ *
+ * Codex emits `\x1b[?1049h` and clear-scrollback sequences during startup and
+ * on repaint. xterm.js obeys them by switching to the alt buffer (no native
+ * scrollback) and wiping saved lines, so the user's conversation history
+ * disappears on every tab switch / pane refresh. Stripping these from the
+ * replayed byte stream keeps everything in the main buffer with scrollback
+ * intact. Mirrors the live-stream strip in Session._handleTerminalOutput.
+ */
+// eslint-disable-next-line no-control-regex
+const ALT_SCREEN_TOGGLE_PATTERN = /\x1b\[\?(?:47|1047|1049)[hl]/g;
+// eslint-disable-next-line no-control-regex
+const ERASE_SCROLLBACK_PATTERN = /\x1b\[3J/g;
+
 /**
  * Strip redundant Ink spinner/status-bar redraw frames from the terminal buffer.
  * Ink (Claude Code's TUI) uses absolute cursor positioning (CSI n d = VPA) to animate
@@ -916,7 +935,14 @@ export function registerSessionRoutes(
     // During long thinking phases, Ink rewrites the same rows thousands of times
     // (500KB+). Without stripping, tail mode returns only spinner frames and
     // the terminal appears empty when switching tabs.
-    const strippedBuffer = stripInkRedrawBloat(rawBuffer);
+    let strippedBuffer = stripInkRedrawBloat(rawBuffer);
+
+    // Strip alt-screen toggles and scrollback-erase from codex byte streams.
+    // xterm.js obeys them by switching to its scrollback-less alt buffer and
+    // wiping saved lines, so conversation history disappears on tab switch.
+    if (session.mode === 'codex') {
+      strippedBuffer = strippedBuffer.replace(ALT_SCREEN_TOGGLE_PATTERN, '').replace(ERASE_SCROLLBACK_PATTERN, '');
+    }
 
     if (tailBytes > 0 && strippedBuffer.length > tailBytes) {
       // Fast path: tail from the end, skip expensive banner search on full 2MB buffer.