Terminal relay: answer Hammerhead's terminal sessions through the agent

When someone opens a terminal for this site, Hammerhead lists the session in the
site's config. The agent connects out to Hammerhead and joins that connection to
its own SSH server, so no port has to be open on an edge. The agent now reports
where its SSH server listens and that it can relay.

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

README.md

@@ -36,7 +36,7 @@ Settings and secrets can live in a `.env` file instead of the command line; see
 - **Tunnel:** one WireGuard peer per edge, through a userspace network stack (no root, no kernel interface, no driver; works on Windows, macOS and Linux). Its private key never leaves this machine. TCP connections and native UDP datagrams are accepted from the edge and handed to your services.
 - **Docker discovery:** with `-docker`, lists running containers (and their ports and networks) so you can pick one as a target in the dashboard.
 - **Service discovery:** off unless you ask for it. With `-discover`, lists the TCP services listening on this machine (read from `/proc/net/tcp`, so Linux only). With `-discover-subnets`, it also tries the chosen ports on every address of those subnets. Each hit is identified (SSH, SMTP, FTP, HTTP, HTTPS, or the usual service for that port) and reported to Hammerhead as a suggested target; nothing is exposed until you pick one. It is deliberately gentle: only private, loopback or link-local subnets of at most 1024 addresses are accepted, connection attempts are capped (50 per second by default), a sweep runs every 15 minutes, and a service that stays silent gets a single TLS handshake and then one `HEAD /` request, nothing more. At most 500 services are reported. Only scan networks you are allowed to scan.
-- **SSH server:** off unless you ask for it. With `-ssh`, Bamboo runs a small SSH server, so you can open a shell or run a command on this machine from anywhere through a tunnelled port, with no sshd to install. Put the public keys that may sign in in `authorized_keys` in the data folder (one per line, like OpenSSH; Bamboo will not start the server without at least one, and picks up edits without a restart). Then, in Hammerhead, add a TCP port whose target is this site at `127.0.0.1:2222`, and connect with `ssh -p <port> user@<edge address>`. The user name is ignored: sessions run as the user Bamboo runs as, so a Bamboo running as root gives a root shell, and the keys should be treated that way. Public keys only (no passwords), session channels only (no port forwarding, agent forwarding or SFTP), and it listens on the loopback address unless you change `-ssh-listen`. The host key is created in the data folder on first start and its fingerprint is written to the log, so you can check it the first time you connect. Not available on Windows.
+- **SSH server:** off unless you ask for it. With `-ssh`, Bamboo runs a small SSH server, so you can open a shell or run a command on this machine from anywhere through a tunnelled port, with no sshd to install. Put the public keys that may sign in in `authorized_keys` in the data folder (one per line, like OpenSSH; Bamboo will not start the server without at least one, and picks up edits without a restart). Then, in Hammerhead, add a TCP port whose target is this site at `127.0.0.1:2222`, and connect with `ssh -p <port> user@<edge address>`. The user name is ignored: sessions run as the user Bamboo runs as, so a Bamboo running as root gives a root shell, and the keys should be treated that way. Public keys only (no passwords), session channels only (no port forwarding, agent forwarding or SFTP), and it listens on the loopback address unless you change `-ssh-listen`. The host key is created in the data folder on first start and its fingerprint is written to the log, so you can check it the first time you connect. Hammerhead's dashboard terminal reaches this server through the agent itself: Bamboo reports where the server listens, and when someone opens a terminal it connects out to Hammerhead and joins that connection to the server, so no port has to be open anywhere. Add the public key Hammerhead shows (SSH button on the Sites page) to `authorized_keys` to allow it. Not available on Windows.
 - **Mail relay:** for a mail egress configured in the dashboard, opens a small SMTP relay on this machine (default `127.0.0.1:2525`). Point your mail server at it (Postfix: `relayhost = [127.0.0.1]:2525`) and each outgoing message leaves from the edge's public IP, so reverse DNS, SPF and your HELO name line up. Supports `AUTH PLAIN/LOGIN`, requires a password if it listens beyond `127.0.0.1`, relays one destination domain per transaction (extra domains get `452` so your server resends them separately), and passes your server's queue and retries through unchanged. 50 MB message limit.
 
 ## Deploying