Function import_ambient_once 

fn import_ambient_once(ambient_creds: &Path, dest_dir: &Path) -> Result<bool>

Import the user’s ambient credential into the AgentMux dir ONCE — a read-only copy of ~/.<provider>/.credentials.json (never a move, never a write-back to the user’s dir). Sentinel-gated (<dest>/.agentmux-cred-seeded) so a later logout in the AgentMux dir sticks and we never silently re-import. Returns Ok(true) when a copy happened this call. (SPEC_PROVIDER_ISOLATION INV-R.)