A reply can stop and ask you something

Three features turn out to be one mechanism: a command waiting to be
approved, a question the model wants answered, and "this reply is waiting
for you" are all — stop the generation, put an interactive block in the
bubble, wait for a POST, carry on. So there is one primitive, and the only
thing using it so far is `ask_user`: a model can offer you a few answers
and a box to write your own.

The shell executor is not here yet. This lands first on purpose, because
it is the riskiest machinery in the feature and it is worth having working
before any subprocess exists to complicate it.

Two things about where the pause sits. It pauses a round, not a call: a
round's calls run together under a semaphore, and parking four coroutines
on four separate answers inside that gather would queue them behind each
other invisibly. And Stop had to be taught about it — `cancel` is read
between streamed chunks and there are no chunks while paused, so the
button did nothing at all until `request_stop` learned to resolve the
pause itself.

Also here: a risk class on every tool (read, write, execute), which is
what the four permission modes will be a table over, and the systemd unit
loses ProtectKernelTunables. That last one is not tidying — it
bind-mounts /proc/sys read-only, which stops bubblewrap mounting /proc at
all, and the obvious workaround would expose this process's environment
and with it the encryption key.

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

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

@@ -110,6 +110,14 @@
       <div class="tool-activity-list" id="tools-{{ message.id }}"
            sse-swap="tools" hx-swap="innerHTML"></div>
 
+      {# Where a question from the model, or a command waiting to be allowed,
+         lands. Unlike the blocks above it this frame is sent on every version
+         bump including when it is empty, because the card has to disappear the
+         moment it is answered. The buttons inside are hx-post and they work:
+         the SSE extension processes what it swaps in. #}
+      <div class="interaction-slot" id="ask-{{ message.id }}"
+           sse-swap="ask" hx-swap="innerHTML"></div>
+
       {# The server re-renders the answer as Markdown a few times a second and
          replaces this whole block, so formatting appears as the model writes
          rather than snapping into place at the end. #}