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/partials/_chat_link.html

@@ -16,6 +16,17 @@
           {{ '' if chat_item.unread else 'hidden' }} title="New reply"></span>
   </a>
   <span class="nav-item__actions">
+    {# Works from any page carrying the sidebar, not only from inside the chat.
+       The response carries both out-of-band spans, so the heading follows if
+       this happens to be the open chat. #}
+    <button class="btn btn--icon btn--sm" type="button"
+            hx-patch="/api/chats/{{ chat_item.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_item.title }}"
+            aria-label="Rename chat" title="Rename chat">
+      {{ icon("pencil", "icon--sm") }}
+    </button>
     <button class="btn btn--icon btn--sm" type="button"
             hx-delete="/api/chats/{{ chat_item.id }}"
             hx-confirm="Delete “{{ chat_item.title }}”? This cannot be undone."