feat(plumbing): add 7 new interrogation commands to complete plumbing layer

Implements merge-base, snapshot-diff, domain-info, show-ref, check-ignore, check-attr, and verify-object — closing every gap identified in the plumbing audit against Git's full command surface and adding Muse-specific primitives.

Each command: - Emits JSON by default with a stable, versioned schema - Accepts --format text for human-readable output - Has short (-x) and long (--xxx) flag forms on every option - Uses strict exit codes (0 success / 1 user error / 3 internal error) - Is covered by a dedicated test file (7 new files, 72 new tests) - Is documented in docs/reference/plumbing.md

Zero mypy errors, zero typing_audit violations, 2681 tests green.