fix(login): avoid SSH passphrase prompt with ssh-agent logins (#1102)

Fixes #866

## Problem

`tea` still prompts for an SSH key passphrase for logins created with
`--ssh-agent-key` or `--ssh-agent-principal`. During `tea login add`, tea
auto-discovers a matching private key in `~/.ssh` even when the login is
configured to use the running ssh-agent. That on-disk key is stored in
`ssh_key`, so later `Login.Client()` asks for its passphrase and tells the SDK
to load the file from disk instead of signing through the agent.

## Changes

- Stop auto-discovering a private key when the login uses the ssh-agent.
- Add `Login.SSHKeyPath()` and use it for HTTPSign and git auth, so agent
  logins pass an empty key path and the SDK talks to `ssh-agent`.
- Prefer SSH remotes for agent logins, matching key-file logins.
- Add unit tests for the new key-path and auto-discovery behavior.

## Testing

- `go test ./modules/config ./modules/task ./modules/git`
- `go build ./...`

---------

Co-authored-by: bircni <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/tea/pulls/1102
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/config/login_test.go

@@ -90,6 +90,39 @@ func TestLoginClientWithSSHPubkeyDoesNotDeadlockOnFirstRequest(t *testing.T) {
 	assert.EqualValues(t, 1, signedIssueRequests.Load())
 }
 
+func TestLoginSSHKeyPath(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name  string
+		login Login
+		want  string
+	}{
+		{
+			name:  "disk key",
+			login: Login{SSHKey: "/tmp/id_ed25519"},
+			want:  "/tmp/id_ed25519",
+		},
+		{
+			name:  "ssh agent ignores disk key",
+			login: Login{SSHKey: "/tmp/id_ed25519", SSHAgent: true},
+			want:  "",
+		},
+		{
+			name:  "ssh agent without disk key",
+			login: Login{SSHAgent: true},
+			want:  "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			assert.Equal(t, tt.want, tt.login.SSHKeyPath())
+		})
+	}
+}
+
 func writeTestSSHKey(t *testing.T) (string, string) {
 	t.Helper()