feat: store OAuth tokens in OS keyring via credstore (#926)

## Summary

- Introduce `github.com/go-authgate/sdk-go/credstore` to store OAuth tokens securely in the OS keyring (macOS Keychain / Linux Secret Service / Windows Credential Manager), with automatic fallback to an encrypted JSON file
- Add `AuthMethod` field to `Login` struct; new OAuth logins are marked `auth_method: oauth` and no longer write `token`/`refresh_token`/`token_expiry` to `config.yml`
- Add `GetAccessToken()` / `GetRefreshToken()` / `GetTokenExpiry()` accessors that transparently read from credstore for OAuth logins, with fallback to YAML fields for legacy logins
- Update all token reference sites across the codebase to use the new accessors
- Non-OAuth logins (token, SSH) are completely unaffected; no migration of existing tokens

## Key files

| File | Role |
|------|------|
| `modules/config/credstore.go` | **New** — credstore wrapper (Load/Save/Delete) |
| `modules/config/login.go` | Login struct, token accessors, refresh logic |
| `modules/auth/oauth.go` | OAuth flow, token creation / re-authentication |
| `modules/api/client.go`, `cmd/login/helper.go`, `cmd/login/oauth_refresh.go` | Token reference updates |
| `modules/task/pull_*.go`, `modules/task/repo_clone.go` | Git operation token reference updates |

## Test plan

- [x] `go build ./...` compiles successfully
- [x] `go test ./...` all tests pass
- [x] `tea login add --oauth` completes OAuth flow; verify config.yml has `auth_method: oauth` but no token/refresh_token/token_expiry
- [x] `tea repos ls` API calls work (token read from credstore)
- [x] `tea login delete <name>` credstore token is also removed
- [x] Existing non-OAuth logins continue to work unchanged

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: https://gitea.com/gitea/tea/pulls/926
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Bo-Yi Wu <appleboy.tw@gmail.com>
Co-committed-by: Bo-Yi Wu <appleboy.tw@gmail.com>

go.mod

@@ -12,6 +12,7 @@ require (
 	github.com/adrg/xdg v0.5.3
 	github.com/araddon/dateparse v0.0.0-20210429162001-6b43995a97de
 	github.com/enescakir/emoji v1.0.0
+	github.com/go-authgate/sdk-go v0.2.0
 	github.com/go-git/go-git/v5 v5.17.0
 	github.com/muesli/termenv v0.16.0
 	github.com/olekukonko/tablewriter v1.1.3
@@ -27,6 +28,7 @@ require (
 )
 
 require (
+	al.essio.dev/pkg/shellescape v1.6.0 // indirect
 	charm.land/bubbles/v2 v2.0.0 // indirect
 	charm.land/bubbletea/v2 v2.0.2 // indirect
 	dario.cat/mergo v1.0.2 // indirect
@@ -52,6 +54,7 @@ require (
 	github.com/cloudflare/circl v1.6.3 // indirect
 	github.com/cpuguy83/go-md2man/v2 v2.0.7 // indirect
 	github.com/cyphar/filepath-securejoin v0.6.1 // indirect
+	github.com/danieljoos/wincred v1.2.3 // indirect
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/davidmz/go-pageant v1.0.2 // indirect
 	github.com/dlclark/regexp2 v1.11.5 // indirect
@@ -61,6 +64,7 @@ require (
 	github.com/go-fed/httpsig v1.1.0 // indirect
 	github.com/go-git/gcfg v1.5.1-0.20230307220236-3a3c6141e376 // indirect
 	github.com/go-git/go-billy/v5 v5.8.0 // indirect
+	github.com/godbus/dbus/v5 v5.2.2 // indirect
 	github.com/golang/groupcache v0.0.0-20241129210726-2c02b8208cf8 // indirect
 	github.com/gorilla/css v1.0.1 // indirect
 	github.com/hashicorp/go-version v1.8.0 // indirect
@@ -87,6 +91,7 @@ require (
 	github.com/xo/terminfo v0.0.0-20220910002029-abceb7e1c41e // indirect
 	github.com/yuin/goldmark v1.7.16 // indirect
 	github.com/yuin/goldmark-emoji v1.0.6 // indirect
+	github.com/zalando/go-keyring v0.2.6 // indirect
 	golang.org/x/net v0.49.0 // indirect
 	golang.org/x/sync v0.19.0 // indirect
 	golang.org/x/text v0.34.0 // indirect