@Copilot implement model-fabric release readiness facade command

[mdheller]

@Copilot takeover requested.

Open a bounded PR adding a release readiness facade command.

Target command:

prophet release readiness model-fabric

Scope:

• add command implementation under internal/cmd
• emit deterministic JSON
• report dimensions: dry-run, checksum, SBOM, provenance, Homebrew formula, stable artifact
• update docs/SUITE_COMMAND_SURFACE.md
• add validation coverage in Makefile or tests

Acceptance criteria:

• make validate remains green
• command does not require network access
• output does not claim production readiness

Do not:

• do not call GitHub APIs
• do not publish releases
• do not claim stable release readiness