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/task/login_create.go

@@ -153,7 +153,7 @@ func CreateLogin(ctx stdctx.Context, name, token, user, passwd, otp, scopes, ssh
 	// so we just use the host
 	login.SSHHost = serverURL.Host
 
-	if len(sshKey) == 0 {
+	if shouldFindSSHKey(sshKey, sshAgent) {
 		login.SSHKey, err = findSSHKey(ctx, client)
 		if err != nil {
 			fmt.Printf("Warning: problem while finding a SSH key: %s\n", err)
@@ -176,6 +176,10 @@ func CreateLogin(ctx stdctx.Context, name, token, user, passwd, otp, scopes, ssh
 	return nil
 }
 
+func shouldFindSSHKey(sshKey string, sshAgent bool) bool {
+	return sshKey == "" && !sshAgent
+}
+
 func shouldCheckTokenUniqueness(token string, sshAgent bool, sshKey, sshCertPrincipal, sshKeyFingerprint string) bool {
 	if sshAgent || sshKey != "" || sshCertPrincipal != "" || sshKeyFingerprint != "" {
 		return false