refactor: add credential and Git packages (#1123)

- `internal/auth`: credentials in the OS keyring or a private file, OAuth login with token refresh, SSH HTTP-signature signers (key files with passphrase prompt, ssh-agent keys and certificates), and the Git credential helper protocol.
- `internal/git`: cancellable Git subprocesses, repository and remote discovery (tracked remote via `branch.<name>.remote`), PR checkout by full ref, AGit push, branch cleanup.

Nothing imports these packages yet.

Reviewed-on: https://gitea.com/gitea/tea/pulls/1123
Reviewed-by: TheFox0x7 <95654+thefox0x7@noreply.gitea.com>

internal/auth/gitcredential_test.go

@@ -0,0 +1,54 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package auth
+
+import (
+	"strings"
+	"testing"
+
+	"gitea.dev/tea/internal/settings"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGitCredentialRoutingSeparatesInstallations(t *testing.T) {
+	hosts := map[string]settings.Host{"https://example.test": {}, "https://example.test/gitea": {}, "https://example.test/gitea/nested": {}, "https://example.test:8443/gitea": {}}
+	for _, tc := range []struct{ host, path, pinned, want string }{
+		{"example.test", "gitea/owner/repo.git", "", "https://example.test/gitea"},
+		{"example.test:443", "gitea/owner/repo.git", "", "https://example.test/gitea"},
+		{"example.test", "gitea/nested/owner/repo.git", "", "https://example.test/gitea/nested"},
+		{"example.test", "gitea/nested/owner/repo.git", "https://example.test/gitea", ""},
+		{"example.test", "gitea-other/owner/repo.git", "", "https://example.test"},
+		{"example.test", "", "", ""},
+		{"example.test:8443", "gitea/owner/repo.git", "", "https://example.test:8443/gitea"},
+		{"other.test", "gitea/owner/repo.git", "https://example.test/gitea", ""},
+	} {
+		got, err := MatchGitInstance(GitRequest{Protocol: "https", Host: tc.host, Path: tc.path}, hosts, tc.pinned, "")
+		require.NoError(t, err)
+		require.Equal(t, tc.want, got)
+	}
+	for _, path := range []string{"gitea/../other", "gitea/%2e%2e/other", "gitea%2fother", "gitea/%252e%252e/other", `gitea\other`} { //nolint:misspell // The backslash is an intentionally invalid path separator.
+		_, err := MatchGitInstance(GitRequest{Protocol: "https", Host: "example.test", Path: path}, hosts, "", "")
+		require.Error(t, err, path)
+	}
+	got, err := MatchGitInstance(GitRequest{Protocol: "http", Host: "example.test", Path: "gitea/owner/repo.git"}, hosts, "", "")
+	require.NoError(t, err)
+	require.Empty(t, got)
+}
+
+func TestReadGitCredentialRequestRejectsAmbiguityWithoutEchoingSecrets(t *testing.T) {
+	request, err := ReadGitRequest(strings.NewReader("protocol=https\nhost=example.test:8443\npath=gitea/owner/repo.git\nusername=alice\npassword=discarded-secret\n\n"))
+	require.NoError(t, err)
+	require.Equal(t, GitRequest{"https", "example.test:8443", "gitea/owner/repo.git", "alice"}, request)
+	request, err = ReadGitRequest(strings.NewReader("url=https://example.test/gitea/owner/repo.git\n\n"))
+	require.NoError(t, err)
+	require.Equal(t, "gitea/owner/repo.git", request.Path)
+	request, err = ReadGitRequest(strings.NewReader("url=https://alice@example.test/gitea/owner/repo.git\n\n"))
+	require.NoError(t, err)
+	require.Equal(t, "alice", request.Username)
+	for _, input := range []string{"host=secret-one\nhost=secret-two\n", "url=https://alice:secret@example.test/repo\n", "url=https://example.test/repo\nhost=secret.test\n", "host=secret\x00value\n", "password=" + strings.Repeat("secret", 12000) + "\n"} {
+		_, err := ReadGitRequest(strings.NewReader(input))
+		require.Error(t, err)
+		require.NotContains(t, err.Error(), "secret")
+	}
+}