refactor(credstore): embed credential store and drop sdk-go dependency

- Embed the minimal credstore subset used by tea (SecureStore,
  EncryptedFileStore, KeyringStore, FileStore) as modules/credstore so
  external SDK renames can no longer break the build
- Keep the on-disk format fully compatible: AES-256-GCM values with the
  v1: prefix, credentials.json / credentials.json.enc paths, and the
  Token JSON field names are unchanged, verified by a ciphertext fixture
  generated with sdk-go v1.1.0
- Store the keyring master key under a tea-owned account name
- Reuse the existing kernel-level filelock module instead of the
  upstream lockfile protocol, removing a stale-lock race
- Cover roundtrip, keyring-unavailable fallback, and fixture decryption
  with tests using a mocked keyring
- Remove github.com/go-signet/sdk-go and promote
  github.com/zalando/go-keyring to a direct dependency

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

go.mod

@@ -12,13 +12,13 @@ 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-signet/sdk-go v1.1.0
 	github.com/muesli/termenv v0.16.0
 	github.com/olekukonko/tablewriter v1.1.4
 	github.com/skratchdot/open-golang v0.0.0-20200116055534-eef842397966
 	github.com/stretchr/testify v1.11.1
 	github.com/urfave/cli-docs/v3 v3.1.0
 	github.com/urfave/cli/v3 v3.10.1
+	github.com/zalando/go-keyring v0.2.8
 	golang.org/x/crypto v0.54.0
 	golang.org/x/oauth2 v0.36.0
 	golang.org/x/sys v0.47.0
@@ -74,7 +74,6 @@ require (
 	github.com/xo/terminfo v0.0.0-20220910002029-abceb7e1c41e // indirect
 	github.com/yuin/goldmark v1.8.2 // indirect
 	github.com/yuin/goldmark-emoji v1.0.6 // indirect
-	github.com/zalando/go-keyring v0.2.8 // indirect
 	golang.org/x/exp v0.0.0-20260410095643-746e56fc9e2f // indirect
 	golang.org/x/net v0.56.0 // indirect
 	golang.org/x/sync v0.22.0 // indirect