feat(terminal): newline chord as registry data, plus a Key tester in Settings

capabilities.newline replaces choosing the Shift+Enter bytes in the send-key
route. Key tester shows the keydown/keypress/keyup a browser reports.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/web/public/index.html

@@ -1822,6 +1822,22 @@
                   </div>
                 </div>
               </div>
+            
+              <div class="set-group">
+                <div class="set-group-head"><h4>Key tester</h4><span class="set-scope">device</span></div>
+                <div class="set-group-body">
+                  <div class="set-row has-field" data-search="key tester keyboard shift enter newline diagnose keydown keypress">
+                    <div class="set-row-text">
+                      <span class="set-row-label">Key tester</span>
+                      <span class="set-row-desc">Click the box and press keys to see what this browser reports (key, code, modifiers) for keydown, keypress and keyup. Useful when a shortcut such as Shift+Enter behaves differently on one device. Nothing is sent to a session.</span>
+                    </div>
+                    <input type="text" id="keyTesterInput" class="set-select" readonly autocomplete="off" spellcheck="false"
+                      placeholder="Click here, then press keys"
+                      onkeydown="app.keyTesterEvent(event)" onkeypress="app.keyTesterEvent(event)" onkeyup="app.keyTesterEvent(event)">
+                  </div>
+                  <pre id="keyTesterLog" class="set-note mono" style="display:none;white-space:pre-wrap" data-i18n-skip></pre>
+                </div>
+              </div>
             </section>
 
             <!-- ══ Header &amp; Panels ═══════════════════════════════════════ -->

src/web/public/settings-ui.js

@@ -1114,6 +1114,27 @@ Object.assign(CodemanApp.prototype, {
     this._updateCheck = null;
   },
 
+  /**
+   * Settings → Terminal & Input → Key tester: prints what the browser reports for each key event.
+   * Read-only and local; it never reaches a session. keypress is shown on purpose: that event is
+   * why a Shift-only Enter used to submit (xterm drops Ctrl/Alt keypresses, not Shift ones).
+   */
+  keyTesterEvent(ev) {
+    const log = document.getElementById('keyTesterLog');
+    if (!log) return;
+    // Never preventDefault on keydown: that suppresses the keypress this panel exists to show.
+    // The field is readonly, so nothing is typed into it either way.
+    const mods = ['ctrlKey', 'shiftKey', 'altKey', 'metaKey'].filter((m) => ev[m]).map((m) => m.replace('Key', ''));
+    const line =
+      `${ev.type.padEnd(8)} key=${JSON.stringify(ev.key)} code=${ev.code || '-'} ` +
+      `mods=${mods.join('+') || 'none'}` +
+      (ev.type === 'keypress' ? ` charCode=${ev.charCode}` : '') +
+      (ev.repeat ? ' (repeat)' : '');
+    const lines = (log.textContent ? log.textContent.split('\n') : []).concat(line);
+    log.textContent = lines.slice(-14).join('\n');
+    log.style.display = 'block';
+  },
+
   _setUpdateResult(html) {
     const el = this.$('updateResult');
     if (el) { el.style.display = 'block'; el.innerHTML = html; }

src/web/routes/session-routes.ts

@@ -107,6 +107,7 @@ import { buildAgentCaseMarker, writeAgentCaseMarker } from '../../agent-case-mar
 import { canUsernameRunPrivilegedCommands, resolveClaudeModeForUsername } from '../../user-store.js';
 import { clampEnvOverridesForOwner } from '../../session-env-clamp.js';
 import { enabledClis, getCli } from '../../config/cli-registry/registry.js';
+import type { NewlineSequence } from '../../config/cli-registry/types.js';
 import { resolveCliLaunchError } from '../../utils/cli-launcher.js';
 import { legacyConfigForMode } from '../../session-cli-registry-bridge.js';
 import { isMultiUserMode } from '../../config/multiuser.js';
@@ -2100,13 +2101,7 @@ export function registerSessionRoutes(
     const body = req.body as Record<string, unknown>;
     const key = typeof body?.key === 'string' ? body.key : '';
 
-    // Map key names to hex byte sequences
-    const KEY_HEX_MAP: Record<string, string[]> = {
-      'S-Enter': ['0a'], // \n (line feed)
-      'C-Enter': ['0a'], // \n (line feed)
-    };
-    const hex = KEY_HEX_MAP[key];
-    if (!hex) {
+    if (key !== 'S-Enter' && key !== 'C-Enter') {
       return createErrorResponse(ApiErrorCode.INVALID_INPUT, `Key not allowed: ${key}`);
     }
 
@@ -2116,6 +2111,18 @@ export function registerSessionRoutes(
       return createErrorResponse(ApiErrorCode.INVALID_INPUT, 'No tmux session');
     }
 
+    // Key names map to hex byte sequences. Ctrl+Enter is always a line feed; Shift+Enter is the
+    // CLI's own newline chord (`capabilities.newline`, default line feed), so a CLI that wants
+    // Esc+Enter declares it in the registry instead of being special-cased here.
+    const NEWLINE_HEX: Record<NewlineSequence, string[]> = {
+      'line-feed': ['0a'], // \n
+      'esc-enter': ['1b', '0d'], // ESC CR, the Alt/Option+Enter chord
+    };
+    const hex =
+      key === 'C-Enter'
+        ? NEWLINE_HEX['line-feed']
+        : NEWLINE_HEX[getCli(session.mode)?.capabilities.newline ?? 'line-feed'];
+
     try {
       // Route through the dedicated Codeman socket — bare `tmux` would target the
       // user's default server and never find this session (same #80 regression class).