Add diagnostic and warning model

[rlippmann]

Explore a first-class diagnostic/warning output model for successful transitions that have noteworthy conditions.

Examples:

• use podman instead of docker when docker is not currently in use.
• remove policy docker when docker is already absent.

Questions to resolve:

• Should diagnostics be part of Decision or a separate result type?
• Are warnings advisory only?
• How does this affect Python/TypeScript API compatibility?

Do not assume diagnostics are required; this tracks the design decision.