#3242 fix(cliproxy): make Kamino model families exclusive

Changes

• Make declared Kamino model families exclusive across credentials.
• Keep credentials with the same model_family eligible for that family.
• Exclude claimed families from unassigned credentials while preserving operator-authored exclusions.

Testing

• shellspec spec/cliproxyapi_spec.sh spec/kamino_fleet_spec.sh (99 examples, 0 failures)
• Nix evaluation/build of the Kamino package
• Bash syntax, ShellCheck, nixfmt, and diff checks

Follow-up to #3241.

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Summary by cubic

Makes Kamino model families exclusive across credentials in cliproxy so a model family claimed by one credential is excluded from every other credential.

• A credential with model_family set still excludes the other families and remains eligible for its own family.
• Credentials without a model_family now receive exclusions for every family claimed by other credentials.
• Operator-authored exclusions in existing auth files are preserved alongside the new generated ones.
• Unknown model families still fail the render.
• Updates the spec to cover two credentials sharing a family and one unassigned credential.