RFC 0011: Implement Linux Secret Service backend for profile secrets

[rmanibus]

Context

RFC 0011 proposes native secret storage support. This issue tracks the Linux backend.

Scope

• Implement secret-service://collection/item resolver backend.
• Integrate with resolver routing and typed error model.
• Treat missing DBus/keyring daemon as backend-unavailable (not crash).

Acceptance Criteria

• secret-service://... refs resolve when Secret Service is available.
• Missing session/keyring returns clear fallback guidance (env://...).
• Linux-specific tests are present and skip cleanly when backend is unavailable.
• No secret values are logged or printed.

Out of Scope

• KWallet support (unless explicitly added to scope later).
• macOS/Windows backend work.

[rmanibus]

Parent epic: #93
RFC: https://github.com/Cloudstic/cli/blob/main/rfcs/0011-profile-credential-storage-backends.md