A terminal panel beside an agent chat

A real shell on the chat's own connection, opened and closed like the
inspector and never beside it. The modes govern the model; what a person
types is theirs, since they hold the credential and could open the same
shell with an ssh client. The model cannot see the panel -- a button
copies the output you choose into the composer.

The session outlives the socket: closing the panel leaves a build
running, and coming back reattaches with the scrollback. Two tabs share
one shell and the smaller window decides the size. It ends on an idle
timeout, on deleting the chat, on disabling, moving or deleting the
connection, and on a restart -- which says why rather than quietly
opening a fresh shell that has lost the working directory.

The nginx template's `Connection ""` is right for SSE and fails every
WebSocket handshake, so `location /` now uses a `map $http_upgrade`;
update.sh grows a drift check for it, because the only symptom on a
stale vhost is a panel that cannot connect.

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

PLAN.md

@@ -112,6 +112,14 @@ be a different project, not a refactor.
       switches to Edit and sends it back quoted rather than as an instruction
 - [x] Per-reply budgets on steps, wall clock and output, with time spent
       waiting for you subtracted
+- [x] **A terminal panel** beside the chat, holding a real shell on that chat's
+      own connection. The modes govern the model; what a person types is theirs,
+      since they hold the credential and could open the same shell with an ssh
+      client. The model cannot see the panel — sending it output is a button
+- [x] The shell outlives the panel and the page: closing it leaves a build
+      running, and coming back reattaches with the scrollback. An idle timeout
+      is what eventually ends one, and so does deleting the chat, or disabling,
+      moving or deleting the connection
 
 ### The library
 - [x] **Knowledge bases** — documents, images and saved web pages, grouped into