feat(auth): add token cache with thread-safe auto-refresh

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

internal/slurm/token_source.go

@@ -0,0 +1,8 @@
+package slurm
+
+import "context"
+
+// TokenSource provides tokens for authentication.
+type TokenSource interface {
+	Token(ctx context.Context) (string, error)
+}