PLC8: publish the Plugin author SDK and converge managed Skill actions

[zhnt]

Objective

After PLC7 is complete and its three-view review findings are fixed, freeze and publish the minimum public Plugin author SDK and converge native/package managed Skill actions over the single Resource Catalog.

Hard prerequisite

• PLC7: pluginize coding.arch.default as the second Capability Provider #507 is closed as completed.
• PLC7 architecture, correctness/security, and Product/test reviewers have re-reviewed every fix.
• No PLC8 runtime/public export work begins before that gate.

Ordered scope

1. Freeze declaration IR and engine-feature negotiation against synthetic, LSP, Base, and Arch evidence.
2. Publish the minimum documented author surface under loushang.plugin without owner authority objects.
3. Add inert package validation and a separate explicitly approved execution-conformance command.
4. Preserve each SKILL.md as a resource_item; converge list/enable/load/refresh through one Catalog.
5. Add strict native/package managed Skill-action declarations with exact digest, runtime, argv/cwd/env/effects/containment and Approval evidence.
6. Add compatibility fixtures for two IR/engine versions, author documentation, and a non-example conformance package.

Exit gate

• no public Graph, registry, scope, approval store, mutable Plugin context, or owner authority leaks;
• validation never imports or executes Plugin code;
• Skill content is revision/digest reconstructible;
• native and packaged actions prove containment, Approval use, and revision pinning;
• three independent review views pass and every blocking finding is fixed/re-reviewed before PLC9 starts.

Canonical plan: docs/internals/architecture/harness/plugin/plugin-lifecycle-coding-pluginization-plan.md.

This issue is Product/Harness work only. It does not authorize MCP expansion, marketplace work, remote-service shortcuts, or a second Plugin/Graph/Resource authority.