[ctawiah]

Context

Formats need one registry, path grammar, source update engine, and first independent adapters.

What changes

• Add external adapter registration.
• Add exact select and map paths.
• Add Markdown and Agent Skill adapters.
• Add neutral fixtures, conformance tests, and the fixture path guard.

sections[heading="Support"].body

Design notes

• Adapters receive bytes only.
• Existing repository guidance remains unchanged.
• Tests create real skill paths only in temporary directories.

Review focus

• Can an external package register an adapter?
• Do edits preserve bytes outside accepted ranges?

Verification

• make build
• go test ./...
• Repository lint
• Relevant race tests

Related changes

1. ctawiah/sync-projection-core-3bbf: projection and problem core
2. This PR: reference registry and Markdown
3. ctawiah/sync-structured-adapters-3bbf: structured adapters
4. ctawiah/sync-provider-profiles-3bbf: provider profiles
5. ctawiah/sync-python-adapter-3bbf: Python adapter
6. ctawiah/sync-typescript-adapters-3bbf: TypeScript and JavaScript adapters
7. ctawiah/sync-descriptor-storage-3bbf: descriptor storage
8. ctawiah/sync-source-publication-3bbf: source compilation and publication
9. ctawiah/sync-source-planner-3bbf: planner integration
10. ctawiah/sync-descriptor-detach-3bbf: descriptor detach
11. ctawiah/sync-link-preparation-3bbf: link preparation
12. ctawiah/sync-link-command-3bbf: link commands
13. ctawiah/sync-reference-cleanup-3bbf: Phase 1 cleanup