Conciseness

The ~25-line body is lean and directive with zero padding: every line either points to the source of truth, states a constraint, or defines the rebuild/promote workflow. It assumes competence and explicitly states "Keep this skill thin so the guide remains the audited source of truth", so nothing could be trimmed without losing information.

Actionability

Guidance is concrete for an instruction-only skill: exact paths ("tools/tart_macos/README.md", "tools/tart_macos/packer/frb-tart-base.pkr.hcl"), named artifacts ("frb-tart-base-candidate", "frb-tart-base"), and specific dos/don'ts. Not a 5 because no actual commands (packer build, smoke-test invocation) appear inline — they are deferred to the external README; not a 3 because the specific paths, VM names, and rules make the guidance directly executable in outline.

Workflow Clarity

The durable-change workflow is clearly sequenced with a validation gate: "Use `frb-tart-base-candidate` for rebuilds, smoke test the candidate, then promote it to `frb-tart-base`", plus a guardrail against mutating the promoted base. Not a 5 because there is no error-recovery loop (what to do when the smoke test fails — presumably covered in the README); not a 3 because the sequence and the smoke-test checkpoint are explicit, not implicit.

Progressive Disclosure

This is a sub-50-line, single-purpose skill with well-organized sections and no bundle files (references/, scripts/, assets/ are absent), so the simple-skill exception applies. Its one clearly signaled reference (the checked-in README) is one level deep, and it cleanly routes the Tart daily-usage concern to the sibling skill.

Specificity

The description names a concrete domain ("FRB Tart base VM for iOS Simulator validation") and enumerates specific areas — "local OCI registry mirroring, Packer provisioning, and base VM hygiene" — which is several specific items, though the actions themselves ("preparing or explaining") are generic verbs. Not a 5 because it omits concrete operations from the body's workflow (rebuild, smoke test, promotion), and not a 3 because the enumerated coverage goes beyond 1-2 actions.

Completeness

It has an explicit "Use when preparing or explaining..." trigger clause plus a clear 'what' via the enumerated scope (registry mirroring, Packer provisioning, hygiene). Not a 5 because the 'what' is expressed as included topics rather than explicit capability statements, and the 'when' could name more concrete trigger scenarios; not a 3 because both what and when are present and specific.

Trigger Term Quality

Strong natural keyword coverage for this domain ("Tart base VM", "iOS Simulator", "OCI registry", "Packer", "provisioning") that a user in this repo would plausibly say. Not a 5 because synonyms and variations like "rebuild", "promote", "macOS VM image", or file/tool extensions are missing; not a 3 because coverage clearly goes beyond a couple of generic keywords.

Distinctiveness Conflict Risk

"FRB Tart base VM" is a highly niche, named target with distinct technical triggers (OCI registry, Packer) that would not collide with generic skills; it only brushes against the sibling frb-dev-env skill, which the body explicitly disambiguates. Clear niche with minimal conflict risk.