sshd: implement --ssh.enable with a pure-Go OpenSSH server

[ananthb]

Summary

• --ssh.enable was a stub (slog.Info(\"ssh requested\")), so rescue sessions couldn't actually connect. Now it runs a pure-Go OpenSSH server (golang.org/x/crypto/ssh + github.com/creack/pty) inside the pivoted rootfs.
• Auth: public keys from --ssh.authorized-keys (standard authorized_keys format, one per line; blank + comment lines skipped) OR password from --ssh.password. Both accepted when both set; server refuses to start with neither.
• Sessions: pty-req + shell → interactive /bin/sh -l in a real pty (creack/pty); exec → non-pty /bin/sh -c \"…\". TERM, env requests, window-change all propagated.
• Ephemeral ed25519 host key generated at start; fingerprint logged. All sessions run as root — the pivoted rootfs has no user db.
• Dry-run output now shows the SSH step + configured auth methods.

Linux-only; non-Linux stub returns "not supported" (pty + os/exec shell handling needs Linux semantics).

Motivation

Ran a real pivot into a coreos-installer image today. The tsnet node registered on the tailnet but SSH timed out — tsnet doesn't ship the Tailscale SSH server, and xmorph's own --ssh.enable was a no-op. No way to observe the install; had to power-cycle when things went sideways.

Test plan

• go test ./internal/postpivot/... ./internal/config/... ./internal/cli/... on Linux (endeavour): all pass.
• New tests cover: pubkey exec (echo hello), exit status propagation, password auth, unknown-key rejection, missing-auth refusal, multi-line authorized_keys parsing (with blank + comment lines), bad-line error surfacing.
• Real hardware: retry the enterprise Flatcar pivot with --ssh.enable --ssh.authorized-keys ...; confirm we can SSH into the pivoted env and observe install progress.

Follow-ups (out of scope)

1. --ssh.tailscale-auth: bind on the tsnet listener and use LocalClient().WhoIs() to identify the peer, trust any tailnet member (no ACL parsing yet).
2. Full tailnet SSH-policy enforcement — duplicates a big chunk of tailscaled; only worth it if someone actually needs it.
3. Optional persistent host key (write to /etc/xmorph/ssh_host_ed25519_key) so known_hosts entries survive multi-pivot fleets.