Add typed SSH remote endpoints

docs/git-synchronization.md

@@ -20,13 +20,22 @@ committer. Only their affected paths are staged, unrelated index state is
 preserved, no-op mutations create no commit, and commit failures restore the
 index so the storage transaction can roll back its files.
 
+## Remote endpoint contract
+
+Storage parses credential-free HTTPS URLs plus feature-gated `ssh://` and
+scp-like SSH URLs into one typed endpoint contract. Local paths, `git://`,
+`file://`, helper transports, URL rewrites, separate push URLs, embedded
+credentials, and unknown schemes are rejected before transport. A build
+without the `ssh` feature reports SSH as unsupported before connection or
+repository mutation instead of treating its configuration as malformed.
+
 ## HTTPS transport
 
-Remote URLs must be absolute, credential-free HTTPS URLs. SSH, scp syntax,
-`git://`, `file://`, local paths, helper transports, URL rewrites, separate push
-URLs, and unknown schemes are rejected before transport. Credentials are
-requested with the configured server ID and application ID and remain outside
-Git configuration.
+HTTPS credentials are requested with the configured server ID and application
+ID and remain outside Git configuration. The SSH session, host-verification,
+authentication, and pack-protocol implementations are separate milestone work;
+until those layers are present, network operations on SSH endpoints return the
+typed unsupported-transport result.
 
 Fetch uses the embedded Rust smart-HTTP client with an explicit credential
 callback, so Git's credential cascade is never entered. Push implements the