Bitwarden Secrets Manager integration replaces per-provider API keys

What this changes for operators

• Operators managing credentials must decide whether to migrate from per-provider API keys to centralized Bitwarden Secrets Manager, changing where secrets live and how they rotate.
• Centralized secret management enables rotation and revocation that scattered per-provider keys did not; an operator wiring CI/automation must re-point credential sourcing.
• Verification path: configure Bitwarden Secrets Manager on v0.15.0, confirm the agent resolves credentials from it, and test a rotation to verify the agent picks up the new secret.