CLI exit codes

The CLI is part of the public surface. Exit codes are intentional and stable:

• 0: success.
• 2: validation or invariant failure (misuse, contract violation).
• 3: backend capability or availability failure (e.g., ND without ANN).
• 4: budget exhaustion or policy refusal.
• 1: unexpected internal error (treated as a bug).

There are no silent retries or fallbacks; failures are terminal and mapped to the nearest code above.