Web terminal: open a shell on a site from the dashboard

Hammerhead signs in to the site's Bamboo SSH server through the SSH port an edge
exposes, with a key of its own, and relays the shell to the browser over a
WebSocket (xterm.js, served by Hammerhead). The SSH dialog shows Hammerhead's
public key to add to authorized_keys. Host keys are saved on first use and a
changed one is refused; the terminal opens only from the dashboard's own
origin, is audited, and closes after 30 minutes idle. Errors reach the page as
messages over the socket. An end-to-end test runs a real shell through the
edge, tunnel and Bamboo.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

web/term/README.txt

@@ -0,0 +1,3 @@
+The web terminal uses xterm.js 6.0.0 and its fit addon 0.11.0 (MIT license, see LICENSE-xterm.txt),
+copied from the npm packages @xterm/xterm and @xterm/addon-fit. The only change is that the trailing
+source map comment is removed, because the map files are not shipped.