cli, crypto: Comment keys in SSH Agent

To avoid confusion how the key ended up in SSH Agent, add a comment that
adds some explanation.

See also
<https://radicle.zulipchat.com/#narrow/channel/392584-Feedback/topic/ssh-key.20with.20comment.20in.20agent/with/618208959>.

crates/radicle-cli/src/commands/auth.rs

@@ -184,8 +184,9 @@ pub fn register(
     profile: &Profile,
     passphrase: crypto::ssh::Passphrase,
 ) -> anyhow::Result<()> {
-    let secret = profile
-        .keystore
+    let keystore = &profile.keystore;
+    let secret_key_path = keystore.secret_key_path();
+    let secret = keystore
         .secret_key(Some(passphrase))
         .map_err(|e| {
             if e.is_crypto_err() {
@@ -194,9 +195,12 @@ pub fn register(
                 e.into()
             }
         })?
-        .ok_or_else(|| anyhow!("Key not found in {:?}", profile.keystore.secret_key_path()))?;
+        .ok_or_else(|| anyhow!("Key not found in '{}'", secret_key_path.display()))?;
 
-    agent.register(&secret)?;
+    agent.register(
+        &secret,
+        format!("Radicle key loaded from '{}'.", secret_key_path.display()),
+    )?;
 
     Ok(())
 }

crates/radicle-crypto/src/ssh/agent.rs

@@ -93,7 +93,7 @@ impl Agent {
     }
 
     /// Register a key with the agent.
-    pub fn register(&mut self, key: &SigningKey) -> Result<(), AgentError> {
+    pub fn register(&mut self, key: &SigningKey, comment: String) -> Result<(), AgentError> {
         use ssh_key::private::{Ed25519Keypair, KeypairData};
         self.client.add_identity(proto::AddIdentity {
             credential: PrivateCredential::Key {
@@ -103,7 +103,7 @@ impl Agent {
                     )
                     .unwrap(),
                 ),
-                comment: "".into(),
+                comment,
             },
         })
     }