Report drift between .github/labels.yaml with .meta/label-map.json

[williamthorsen]

Problem

.meta/label-map.json maps commit prefixes to GitHub label names, but there's no check that those labels actually exist in .github/labels.yaml. A missing label causes gh pr create to fail silently in the delegate skill.

Acceptance criteria

• A command or check compares label names in .meta/label-map.json against entries in .github/labels.yaml and reports any that are defined in the map but missing from the labels file
• Output includes actionable guidance (e.g., the YAML entry to add)

[williamthorsen]

Updating scope based on a design discussion: the label-map.json schema and generation are moving to @williamthorsen/release-kit (tracked in williamthorsen/node-monorepo-tools#322). With release-kit owning generation of both .github/labels.yaml and .meta/label-map.json in the same pass:

• Derivative drift between the two files becomes structurally impossible unless someone hand-edits one.
• The right remediation for any drift (including hand-edits) is to rerun generate rather than to invoke a dedicated check tool.
• Running generate in CI doubles as the drift check: idempotent when in sync, modifying writes (or syncing to GitHub) when not.

Proposing this issue close as resolved-by-design once node-monorepo-tools#322 ships. The companion cleanup of removing the schema from this repo is tracked in #469.