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/repo_clone.go

@@ -33,7 +33,7 @@ func RepoClone(
 		return nil, err
 	}
 
-	auth, err := local_git.GetAuthForURL(originURL, login.GetAccessToken(), login.SSHKey, callback)
+	auth, err := local_git.GetAuthForURL(originURL, login.GetAccessToken(), login.SSHKeyPath(), callback)
 	if err != nil {
 		return nil, err
 	}
@@ -68,7 +68,7 @@ func RepoClone(
 
 func cloneURL(repo *gitea.Repository, login *config.Login) (*url.URL, error) {
 	urlStr := repo.CloneURL
-	if login.SSHKey != "" {
+	if login.SSHKeyPath() != "" || login.SSHAgent {
 		urlStr = repo.SSHURL
 	}
 	return local_git.ParseURL(urlStr)