docs(plans): add Agent Plugins v1.0.0 manifest conformance plan

[kieranklaassen]

Summary

Adds the implementation plan for conforming the repo-root plugin package to the Agent Plugins v1.0.0 specification, produced from the 2026-08-06 audit. Refs #1341.

The plan covers three units: swap root plugin.json's $schema to the canonical Agent Plugins identifier and mirror permitted metadata (U1), guard manifest conformance in CI by tightening the existing release-metadata checks (U2), and document the compatibility posture — including the deliberate skill-frontmatter gap on strict clients — in docs/specs/ plus a user-facing README note (U3).

Key decisions captured in the plan

• Root plugin.json serves both Antigravity and Agent Plugins clients with one $schema value; agy tolerance is gated by a manual verification step with a pre-approved revert path (the empirical agy probe never tested a foreign $schema value).
• Skill frontmatter (argument-hint, disable-model-invocation) stays as-is: relocating it would regress Claude Code behavior. Strict Agent Plugins clients loading 4 of 32 skills is a disclosed posture; converter-layer conformant emission is deferred follow-up work.
• Grok Build also resolves the root manifest at runtime (surfaced during document review), so it joins the consumer inventory and gets a belt-and-suspenders manual gate.

Validation

Plan-only change — no code or skill content touched. The plan passed ce-plan's confidence check and a four-persona ce-doc-review pass (coherence, feasibility, product-lens, adversarial); all five actionable findings were folded into the document.

Security Disclosure

No security-relevant changes.

Agent Disclosure

• Model: Cursor Cloud Agent · Fable 5