A terminal panel beside an agent chat

A real shell on the chat's own connection, opened and closed like the
inspector and never beside it. The modes govern the model; what a person
types is theirs, since they hold the credential and could open the same
shell with an ssh client. The model cannot see the panel -- a button
copies the output you choose into the composer.

The session outlives the socket: closing the panel leaves a build
running, and coming back reattaches with the scrollback. Two tabs share
one shell and the smaller window decides the size. It ends on an idle
timeout, on deleting the chat, on disabling, moving or deleting the
connection, and on a restart -- which says why rather than quietly
opening a fresh shell that has lost the working directory.

The nginx template's `Connection ""` is right for SSE and fails every
WebSocket handshake, so `location /` now uses a `map $http_upgrade`;
update.sh grows a drift check for it, because the only symptom on a
stale vhost is a panel that cannot connect.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/web/templates/chat/_terminal.html

@@ -0,0 +1,41 @@
+{% from "_macros.html" import icon %}
+{#
+  The terminal panel: a fourth child of .shell, to the left of the inspector and
+  never open beside it. Empty on a page load -- xterm is created the first time
+  the panel is opened, so the 280KB it costs is paid by somebody who asked for a
+  shell rather than by everyone who opened a chat.
+
+  What is typed here is not run past the chat's mode or its allow and deny
+  lists. Those govern the model, which reads pages and files it did not write;
+  the person at the keyboard holds the credential and could open the same shell
+  with an ssh client.
+#}
+<aside class="terminal" id="terminal" hidden aria-label="Terminal"
+       data-terminal
+       data-url="/api/chats/{{ chat.id }}/terminal/ws"
+       data-label="{{ agent_profile.name if agent_profile else 'this connection' }}"
+       data-dir="{{ chat.project_dir }}">
+  <div class="terminal__header">
+    <h2 class="terminal__title">
+      {{ icon("terminal", "icon--sm") }}
+      <span>{{ agent_profile.name if agent_profile else "Terminal" }}</span>
+      <span class="terminal__where" data-terminal-where>{{ chat.project_dir }}</span>
+    </h2>
+    <button class="btn btn--icon btn--sm" type="button" data-terminal-send
+            title="Put the selection, or the last of the output, into the message box"
+            aria-label="Send to chat">
+      {{ icon("arrow-up", "icon--sm") }}
+    </button>
+    <button class="btn btn--icon btn--sm" type="button" data-toggle="#terminal"
+            aria-label="Close terminal">
+      {{ icon("x", "icon--sm") }}
+    </button>
+  </div>
+
+  <div class="terminal__screen" data-terminal-screen></div>
+
+  <div class="terminal__status">
+    <span class="terminal__message" data-terminal-message>Connecting…</span>
+    <span>Ctrl+Shift+C / V</span>
+  </div>
+</aside>