Names that fit the chat, and a way to change one

Two things about titles were wrong. Every chat spent a second completion on
its name, including an agent chat whose opening words are already a title --
somebody starting one states an objective, not a topic. An agent chat now
takes `fallback_title` from its first prompt and makes no request at all;
an ordinary chat, which opens with a question whose *answer* is what makes a
title worth asking for, is unchanged.

And renaming existed only as the `/title` slash command, which set the heading
and left the sidebar row showing the old name until the next reload -- a rename
that looks half-applied is one people do twice. There are pencil buttons on the
heading and on every sidebar row now, both PATCHing the route that was already
there, and `update_chat` answers a rename with the out-of-band pair the `done`
frame has always sent, so one response moves both. Only on a rename: sending it
for every PATCH would overwrite the heading from an unrelated save. `/title`
sets both spans itself, being a bare fetch rather than htmx.

The dialog is the `data-prompt` mechanism the folder work added, which is why
the heading keeps a button rather than becoming an inline field: it sits in a
flex row beside the badges and the connection chip, and swapping it for a text
box moves all of them.

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

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

@@ -25,6 +25,23 @@
 
       <h1 class="topbar__title">
         <span id="chat-title">{{ chat.title if chat else "New chat" }}</span>
+        {#
+          Rename, where the name is. A themed dialog through `data-prompt`
+          rather than an inline field: the heading is in a flex row beside the
+          badges and the connection chip, and swapping it for a text box moves
+          all of them. The response is the same out-of-band pair the `done`
+          frame sends, so the sidebar row follows without a second request.
+        #}
+        {% if chat %}
+        <button class="btn btn--icon btn--sm topbar__rename" type="button"
+                hx-patch="/api/chats/{{ chat.id }}" hx-swap="none"
+                data-prompt="What should this chat be called?"
+                data-prompt-title="Rename chat" data-prompt-field="title"
+                data-prompt-value="{{ chat.title }}"
+                aria-label="Rename chat" title="Rename chat">
+          {{ icon("pencil", "icon--sm") }}
+        </button>
+        {% endif %}
         {# Beside the title because it describes the chat rather than acting on
            it. The way out of it is Keep, in the overflow menu. #}
         {% if chat and chat.temporary %}