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/services/generation.py

@@ -406,6 +406,13 @@ async def _run(generation: Generation) -> None:
             )
             question = _question_from(payload)
             needs_title = not chat.title_generated
+            # An agent chat is titled from its opening words and never costs a
+            # model call for it. That prompt is a good title already -- somebody
+            # starting one states an objective, not a topic -- while an ordinary
+            # chat opens with a question, whose answer is what makes a title
+            # worth asking for. Read here with the rest, because titling happens
+            # after this session has closed.
+            title_from_prompt = chat.kind == KIND_AGENT
             # Read here, with the rest, because titling happens after this
             # session has closed and must not open another one.
             title_prompt = prompts_service.resolve(db, "task.title")
@@ -714,7 +721,7 @@ async def _run(generation: Generation) -> None:
         # a chat title is never worth surfacing an error for.
         title = ""
         if needs_title and question:
-            if generation.error or endpoint is None:
+            if title_from_prompt or generation.error or endpoint is None:
                 title = chat_service.fallback_title(question)
             else:
                 with contextlib.suppress(Exception):