Implement SSH identity authentication

docs/configuration.md

@@ -70,16 +70,35 @@ The HTTPS account name is stored inside the protected credential record, not in
 TOML. OpenPGP passphrases are addressed by the resolved primary fingerprint.
 
 SSH remotes use either `ssh://[user@]host[:port]/path` or scp-like
-`[user@]host:path` syntax and omit the HTTPS credential fields:
+`[user@]host:path` syntax and omit the HTTPS credential fields. They select
+exactly one private-key file or one already-running SSH-agent identity:
 
 ```toml
 [[git.remotes]]
 name = "origin"
 url = "git@git.example.test:alice/password-store.git"
+ssh_identity_file = "keys/id_ed25519"
+ssh_known_hosts_file = "known_hosts"
 ```
 
+Relative paths are resolved against `config.toml`. If
+`ssh_known_hosts_file` is omitted, `~/.ssh/known_hosts` is used. Agent
+authentication replaces `ssh_identity_file` with an exact SHA-256 fingerprint
+and may name a socket explicitly:
+
+```toml
+ssh_agent_fingerprint = "SHA256:47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU"
+ssh_agent_socket = "/run/user/1000/ssh-agent.socket"
+```
+
+Private-key passphrases are stored by fingerprint in the operating-system
+secret store and never appear in TOML. IronStorage does not read OpenSSH
+configuration, try additional keys, prompt for passwords or
+keyboard-interactive authentication, launch an agent, or invoke proxy/helper
+commands.
+
 The typed endpoint model is always available so an SSH remote remains readable
-in configuration even when the binary was built without SSH. Such a build
+through the Rust API even when the binary was built without SSH. Such a build
 returns a typed unsupported-transport error before connection or repository
 mutation. The optional storage `ssh` feature contains `russh` 0.63.1 and Tokio;
 `russh` default features are disabled and the Ring backend plus RSA key support
@@ -94,6 +113,12 @@ bytes, credentials, queries, fragments, ambiguous unbracketed IPv6 or colon
 paths, leading-option paths, local paths, URL rewrites, separate push URLs,
 helper transports, and unknown schemes fail closed.
 
+Server identity is checked before authentication against the configured
+known-hosts file. Exact, hashed, and non-default-port host entries are
+supported. Unknown keys return their host, algorithm, and SHA-256 fingerprint
+for explicit confirmation; confirming appends the key atomically. A changed
+key is a hard failure and is never replaced by that confirmation API.
+
 `clipboard_timeout_seconds` controls the native clipboard presentation lease.
 It defaults to 45 seconds for upstream `pass` compatibility and must be between
 1 and 300 seconds. The CLI remains alive for the lease so Linux can serve its