feat(framework): adopt the OKF frontmatter vocabulary in aidd_docs

[cduverne]

Problem

Artifacts under aidd_docs/ carry ad-hoc frontmatter (objective, status) with no shared vocabulary, and no directory describes itself. An agent that did not write them cannot tell what they are.

The Open Knowledge Format is a candidate vocabulary. Checked against the Google Cloud announcement: OKF is directories of markdown files with YAML frontmatter, plus one index.md per directory. Each document carries type, title, description, resource, tags, timestamp. Vendor-neutral, no SDK.

So the framework already has the format — markdown with YAML frontmatter, everywhere. What it lacks is the shared vocabulary and the index.

Scope

• Map the OKF fields onto what AIDD artifacts already carry, and name the ones with no equivalent.
• Adopt the vocabulary in the artifacts a foreign agent would consume: specs, PRDs, memory.
• Emit an index.md per aidd_docs/ directory.

Acceptance criteria

• Every artifact a foreign agent would read carries the OKF fields.
• A directory under aidd_docs/ describes itself through its index.md.
• A named tool, outside AIDD, reads one of our artifacts and understands it.

Blocked: no consumer

No OKF reader exists in the AIDD ecosystem today. This buys interoperability with tools nobody has named. The last acceptance criterion is deliberately unsatisfiable right now — that is the point. Priority stays Low until a real consumer appears, and closing this issue is a defensible alternative to doing it.