Registry index, versioning & publish tooling (HAR-1065)

[zhiganov]

Summary

• Adds standalone Node tooling to the registry (no harmonica-mcp dependency — stays runtime-neutral): frontmatter parser, validator with canonical licence sets, publish-guard (licence allowlist + hold flag), and an index generator that emits index.json + a generated README "Methods" table.
• Adds a GitHub Actions workflow running validate + index-sync check + guard on every push/PR.
• Documents the contract in FORMAT.md (tags, hold, versioning convention, the two-repo public/staging publish model).
• Generates the initial index.json + README table.

Test plan

• npm test — 22 unit tests pass (parser, validator, guard, index builder)
• CLI smokes vs fixtures: validate passes 3, guard blocks a held + a non-allowlisted spec, check detects drift
• Full gate green locally: validate + check + guard

Implements the index + versioning + publish-model sub-scope of HAR-1065. Design + plan in docs/plans/2026-06-14-registry-index-{design,plan}.md. The M2M-to-staging relocation and public flip (plan Task 9) are the follow-on rollout, gated on creating the private staging repo.