feat: customizable delete session command template

Symmetric with create template. Default: 'tmux kill-session -t {name}'.
Configurable in Settings → Commands tab (renamed from 'New Session').
DELETE /api/sessions/{name} reads template from settings, substitutes
{name}, runs synchronously via subprocess.run with 30s timeout.

Enables custom teardown workflows like:
  amplifier-dev ~/dev/{name} --destroy

Changes:
- settings.py: add delete_session_template to DEFAULT_SETTINGS
- main.py: DELETE endpoint uses subprocess.run + template instead of run_tmux
- index.html: add delete template textarea + reset btn; rename tab to Commands
- app.js: DELETE_SESSION_DEFAULT_TEMPLATE constant, openSettings loads it,
  bindStaticEventListeners wires input/reset; constant exported

Tests added:
- test_settings.py: 3 tests for default, load, and patch of delete_session_template
- test_api.py: 2 tests for custom/default template substitution via subprocess mock;
  update existing test_delete_session_calls_kill_session to match new implementation
- test_frontend_html.py: 5 tests for textarea, placeholder, rows, reset btn, tab label
- test_app.mjs: 5 tests for constant definition, value, and wiring in openSettings/bindStaticEventListeners

muxplex/frontend/app.js

@@ -142,6 +142,7 @@ const DISPLAY_DEFAULTS = {
   notificationPermission: 'default',
 };
 const NEW_SESSION_DEFAULT_TEMPLATE = 'tmux new-session -d -s {name}';
+const DELETE_SESSION_DEFAULT_TEMPLATE = 'tmux kill-session -t {name}';
 
 // ─── DOM helpers ──────────────────────────────────────────────────────────────
 function $(id) {
@@ -1221,11 +1222,17 @@ function openSettings() {
       autoOpenEl.checked = ss && ss.auto_open !== undefined ? !!ss.auto_open : true;
     }
 
-    // New Session tab - populate template textarea
+    // Commands tab - populate create template textarea
     const templateEl = $('setting-template');
     if (templateEl) {
       templateEl.value = (ss && ss.new_session_template) || NEW_SESSION_DEFAULT_TEMPLATE;
     }
+
+    // Commands tab - populate delete template textarea
+    const deleteTemplateEl = $('setting-delete-template');
+    if (deleteTemplateEl) {
+      deleteTemplateEl.value = (ss && ss.delete_session_template) || DELETE_SESSION_DEFAULT_TEMPLATE;
+    }
   });
 }
 
@@ -1687,7 +1694,7 @@ function bindStaticEventListeners() {
     });
   });
 
-  // New Session tab — template textarea with 500ms debounce
+  // Commands tab — create template textarea with 500ms debounce
   var _templateDebounceTimer;
   on($('setting-template'), 'input', function() {
     clearTimeout(_templateDebounceTimer);
@@ -1697,12 +1704,29 @@ function bindStaticEventListeners() {
     }, 500);
   });
 
-  // New Session tab — reset button restores default template
+  // Commands tab — create template reset button restores default
   on($('setting-template-reset'), 'click', function() {
     var el = $('setting-template');
     if (el) el.value = NEW_SESSION_DEFAULT_TEMPLATE;
     patchServerSetting('new_session_template', NEW_SESSION_DEFAULT_TEMPLATE);
   });
+
+  // Commands tab — delete template textarea with 500ms debounce
+  var _deleteTemplateDebounceTimer;
+  on($('setting-delete-template'), 'input', function() {
+    clearTimeout(_deleteTemplateDebounceTimer);
+    var val = this.value;
+    _deleteTemplateDebounceTimer = setTimeout(function() {
+      patchServerSetting('delete_session_template', val);
+    }, 500);
+  });
+
+  // Commands tab — delete template reset button restores default
+  on($('setting-delete-template-reset'), 'click', function() {
+    var el = $('setting-delete-template');
+    if (el) el.value = DELETE_SESSION_DEFAULT_TEMPLATE;
+    patchServerSetting('delete_session_template', DELETE_SESSION_DEFAULT_TEMPLATE);
+  });
 }
 
 // ─── Test-only helpers ────────────────────────────────────────────────────────
@@ -1798,6 +1822,9 @@ if (typeof module !== 'undefined' && module.exports) {
     createNewSession,
     // Kill session
     killSession,
+    // Constants
+    NEW_SESSION_DEFAULT_TEMPLATE,
+    DELETE_SESSION_DEFAULT_TEMPLATE,
     // Test-only helpers
     _setCurrentSessions,
     _setViewMode,

muxplex/frontend/index.html

@@ -87,7 +87,7 @@
         <button class="settings-tab settings-tab--active" data-tab="display">Display</button>
         <button class="settings-tab" data-tab="sessions">Sessions</button>
         <button class="settings-tab" data-tab="notifications">Notifications</button>
-        <button class="settings-tab" data-tab="new-session">New Session</button>
+        <button class="settings-tab" data-tab="new-session">Commands</button>
       </nav>
       <div class="settings-content">
         <div class="settings-panel" data-tab="display">
@@ -169,6 +169,12 @@
             <span class="settings-helper">{name} is replaced with the session name</span>
             <button id="setting-template-reset" class="settings-action-btn">Reset to default</button>
           </div>
+          <div class="settings-field settings-field--column">
+            <label class="settings-label" for="setting-delete-template">Delete session command</label>
+            <textarea id="setting-delete-template" class="settings-textarea" rows="3" placeholder="tmux kill-session -t {name}"></textarea>
+            <span class="settings-helper">{name} is replaced with the session name</span>
+            <button id="setting-delete-template-reset" class="settings-action-btn">Reset to default</button>
+          </div>
         </div>
       </div>
     </div>

muxplex/frontend/tests/test_app.mjs

@@ -2063,3 +2063,58 @@ test('pollSessions calls updateFaviconBadge', () => {
     'pollSessions must call updateFaviconBadge — update favicon on every poll cycle'
   );
 });
+
+
+// --- Delete session template (task: customizable delete command) ---
+
+test('app.js defines DELETE_SESSION_DEFAULT_TEMPLATE constant', () => {
+  const source = fs.readFileSync(new URL('../app.js', import.meta.url), 'utf8');
+  assert.ok(
+    source.includes('DELETE_SESSION_DEFAULT_TEMPLATE'),
+    'app.js must define DELETE_SESSION_DEFAULT_TEMPLATE constant'
+  );
+});
+
+test('DELETE_SESSION_DEFAULT_TEMPLATE value is tmux kill-session -t {name}', () => {
+  const source = fs.readFileSync(new URL('../app.js', import.meta.url), 'utf8');
+  assert.ok(
+    source.includes("'tmux kill-session -t {name}'") || source.includes('"tmux kill-session -t {name}"'),
+    "DELETE_SESSION_DEFAULT_TEMPLATE must be set to 'tmux kill-session -t {name}'"
+  );
+});
+
+test('openSettings loads delete_session_template from server settings', () => {
+  const source = fs.readFileSync(new URL('../app.js', import.meta.url), 'utf8');
+  const fnStart = source.indexOf('function openSettings(');
+  assert.ok(fnStart !== -1, 'openSettings must exist');
+  const fnEnd = source.indexOf('\nfunction ', fnStart + 1);
+  const fnBody = source.substring(fnStart, fnEnd > fnStart ? fnEnd : fnStart + 3000);
+  assert.ok(
+    fnBody.includes('setting-delete-template'),
+    'openSettings must populate #setting-delete-template from server settings'
+  );
+});
+
+test('bindStaticEventListeners wires delete template input to save', () => {
+  const source = fs.readFileSync(new URL('../app.js', import.meta.url), 'utf8');
+  const fnStart = source.indexOf('function bindStaticEventListeners(');
+  assert.ok(fnStart !== -1, 'bindStaticEventListeners must exist');
+  const fnEnd = source.indexOf('\nfunction ', fnStart + 1);
+  const fnBody = source.substring(fnStart, fnEnd > fnStart ? fnEnd : fnStart + 6000);
+  assert.ok(
+    fnBody.includes('setting-delete-template'),
+    'bindStaticEventListeners must wire #setting-delete-template input event to save'
+  );
+});
+
+test('bindStaticEventListeners wires delete template reset button', () => {
+  const source = fs.readFileSync(new URL('../app.js', import.meta.url), 'utf8');
+  const fnStart = source.indexOf('function bindStaticEventListeners(');
+  assert.ok(fnStart !== -1, 'bindStaticEventListeners must exist');
+  const fnEnd = source.indexOf('\nfunction ', fnStart + 1);
+  const fnBody = source.substring(fnStart, fnEnd > fnStart ? fnEnd : fnStart + 6000);
+  assert.ok(
+    fnBody.includes('setting-delete-template-reset'),
+    'bindStaticEventListeners must wire #setting-delete-template-reset click handler'
+  );
+});