fix(sandbox): pin ssh known_hosts to sandbox id

[pratikbin]

Problem

createos sandbox editor --via tunnel writes HostName 127.0.0.1 for every sandbox. All sandboxes therefore share a single known_hosts identity, [127.0.0.1]:22.

The first sandbox records its host key. The next sandbox presents a different key on the same entry, and ssh reports:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
Offending ED25519 key in ~/.ssh/known_hosts_createos:2

OpenSSH downgrades this to a warning when public-key auth succeeds, so the CLI's own flows keep working and the bug stays invisible. Stricter clients do not. They treat a changed host key as an active attack and refuse to connect.

That makes the second CreateOS sandbox unusable as a remote host for those clients. We hit it while integrating CreateOS sandboxes into Orca, whose SSH relay has a dedicated host-key verification subsystem.

VPN mode has the same defect with a longer fuse: it keys on the overlay IP, and overlay IPs are recycled between sandboxes.

Fix

Add HostKeyAlias <sandbox-id> to both the tunnel and VPN blocks. OpenSSH then keys known_hosts on the sandbox id, which is unique per sandbox and stable across pause/resume, instead of on a shared address.

 Host sb-01m0hp7v5vtdnj2pwpj25drhwe my-box
     HostName          127.0.0.1
+    HostKeyAlias      sb-01m0hp7v5vtdnj2pwpj25drhwe
     Port              22
     User              root

One line per transport. No behaviour change for a single sandbox, and no client-side change needed — HostKeyAlias is standard OpenSSH and is surfaced through ssh -G, so tools that resolve config that way pick it up automatically.

Test plan

• go build ./... passes.
• go test ./cmd/sandbox/ passes.
• New cmd/sandbox/editor_test.go covers the previously untested renderSSHBlock:
  • both transports emit the pin,
  • two sandbox ids produce two distinct host-key identities (the actual regression),
  • an unknown mode still errors.
• Reproduced live against sb-01m0hp7v5vtdnj2pwpj25drhwe; the offending entry was a prior sandbox at known_hosts_createos:2.

Note for reviewers

Existing ~/.ssh/known_hosts_createos files still hold the old 127.0.0.1 entries. They are harmless once this lands, because lookups move to the alias. Deleting that file is safe if you want a clean slate.