nebula-mesh: Decrypted CA private key persists in heap after signing

internal/pki/resolver.go:36-64 constructs a CAManager with the plaintext ed25519.PrivateKey after unwrapping via the master key; internal/pki/ca.go:13-16 stores it. Callers at internal/api/enroll.go:116, internal/api/updates.go:297, and internal/api/mobilebundle.go:40 use the manager for one Sign...