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>

go.mod

@@ -8,6 +8,7 @@ require (
 	git.veillax.com/Veillax/reef v0.1.11
 	github.com/emersion/go-msgauth v0.7.0
 	golang.org/x/crypto v0.58.0
+	golang.org/x/net v0.61.0
 	modernc.org/sqlite v1.61.0
 )
 
@@ -20,7 +21,6 @@ require (
 	github.com/mattn/go-isatty v0.0.24 // indirect
 	github.com/ncruces/go-strftime v1.1.0 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
-	golang.org/x/net v0.61.0 // indirect
 	golang.org/x/sys v0.49.0 // indirect
 	golang.org/x/text v0.43.0 // indirect
 	golang.org/x/time v0.7.0 // indirect