cmd/tailcat: use a short deterministic host for ssh's ControlPath

[korjavin]

Summary

Fixes #12. tailcat ssh passed the raw ConnBlob (the user@<addrblob> argument — potentially 100+ bytes once a full DERP Region is embedded rather than just a RegionID, see ConnInfo in tailcat.go) straight through as the destination hostname to the system ssh client. ssh substitutes that literal hostname into %n for ControlPath (commonly ~/.ssh/master-%r@%n:%p), and the expansion has to fit an AF_UNIX socket path — about 104-108 bytes total, including the home directory. A long ConnBlob blows that budget on its own, so ControlMaster/ControlPath multiplexing failed with unix_listener: path "..." too long for Unix domain socket in an otherwise completely ordinary ssh config, before tailcat was even invoked — exactly the report in #12.

Fix

sshDestHost() gives ssh a short, deterministic pseudo-hostname (an 8-byte SHA-256 prefix of the blob, tailcat-xxxxxxxxxxxxxxxx, 24 bytes fixed width) instead of the raw blob. The real blob is untouched: it still flows into ProxyCommand exactly as before, which is what actually does the connection routing — nothing about how tailcat proxies the connection changes, only the label ssh sees.

Deterministic hashing (not truncation) matters here: ssh's connection sharing keys a control socket off ControlPath, so the same blob has to keep producing the same short host across invocations for multiplexing to correctly reuse — and not collide across — the right server's socket.

Any user@ prefix on the original argument is preserved and now applied to the short host instead of the raw blob, so -l/%r semantics for a real system sshd (via tailcat --serve=22) are unaffected.

Tests

TestSSHDestHost (new) covers:

• deterministic output for the same blob across calls;
• no collision between two distinct blobs;
• fixed-width output regardless of input blob length;
• a realistic ControlPath built from the short host stays comfortably under the ~104-108 byte AF_UNIX limit.

go build ./...
go build -tags ts_omit_ssh ./...
go vet ./...
go test ./...

All pass (default and ts_omit_ssh build tags).

Also manually verified the hash output shape/length against the reporter's math (a real ConnBlob fixture already used elsewhere in this package's tests is 58 bytes; the fixed 24-byte replacement keeps a typical ControlPath well under the limit regardless of how long the underlying blob is, which is the actual point — a blob-length-dependent truncation wouldn't have that guarantee).