A folder that carries something, and a way to name one

A folder was a name and nothing else -- and not even that, since PATCH could
rename one and nothing in the interface ever called it. It now carries a
description, a system prompt, and seeds for the model, the kind and the agent
target, with a settings page behind the row.

The prompt is a fourth rung on the ladder, chat > folder > model > instance,
and it goes above the model deliberately: a model's prompt describes the model
wherever it is used, a folder's describes this piece of work whichever model
is pointed at it. It is read when a reply is built rather than copied when a
chat is made, so editing it reaches the chats already there, and the walk up
the parents is bounded and cycle-safe because it runs on the request path.
`api/pages.py` mirrors the ladder for the settings panel and had to gain the
same rung -- a panel naming the wrong source is worse than one naming none,
because it is believed.

The seeds fill in what the request left empty and nothing it filled in: the
folder says what this work usually needs, the screen in front of somebody says
what they want this time. `ssh_profile_id` is a plain string rather than a
foreign key, for the reason `compacted_through_id` is, so it is validated on
read.

Getting *into* a folder needed fixing too. `/api/chats/start` has accepted a
folder_id since folders existed and nothing ever sent one, so the only route in
was to make the chat elsewhere and move it. There is a New chat here on the row
now, and `?folder=` on the new-chat screen.

Naming is a themed dialog, and deliberately not htmx's hx-prompt: htmx calls
the browser's prompt() synchronously and only then fires htmx:prompt with the
answer already in hand, so intercepting the event cannot supply a different one
and the grey box appears anyway. `data-prompt` follows the data-confirm-button
shape instead -- swallow the click, ask, write the answer into hx-vals,
click again behind a guard. JSON.stringify rather than concatenation, or a
folder called `"` produces hx-vals that does not parse and the rename silently
does nothing. Driven under a DOM stub, and there is a test that no template
brings hx-prompt back.

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

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

@@ -66,6 +66,12 @@
       {% if not chat and starting_temporary %}
       <input type="hidden" name="temporary" value="true">
       {% endif %}
+      {% if not chat and starting_folder %}
+      {# `/api/chats/start` has accepted a folder_id since folders existed and
+         nothing ever sent one, so the only way into a folder was to make the
+         chat elsewhere and move it. This is "New chat here" arriving. #}
+      <input type="hidden" name="folder_id" value="{{ starting_folder.id }}">
+      {% endif %}
 
       {#
         The text, with a mirror behind it.