docs(plugin-spec): limit plugin PR scope to required files

[stephancill]

What

Adds a Contribution Scope section to the plugin spec and marks the registry/inventory tables as maintainer-managed, so a plugin PR's diff is limited to the files actually required.

A plugin contribution should change only:

• the plugin file skills/base-mcp/plugins/<slug>.md, and
• the tag vocabulary list — only when introducing a genuinely new tag.

It should not edit the SKILL.md plugins table (the registry), the Integration Types Examples column, or the Existing Plugin Conformance table/count — maintainers update those when a plugin is accepted.

Why

The spec currently displays the registry and the Existing Plugin Conformance table inline without saying who owns them, so contributors infer they should add themselves. In practice most in-flight plugin PRs add their own SKILL.md row and a conformance-table entry (and bump the plugin count). This:

• self-asserts "native plugin" status (the conformance table and SKILL.md list are framed as Base-maintained), and
• guarantees merge conflicts between concurrent submissions, since they all edit the same table, the same Examples cell, and the same count.

Making the scope explicit (and labelling those tables maintainer-managed) prevents the conflicts and keeps review focused on the plugin file itself.

Changes

• New ## Contribution Scope section under "How to Author a New Plugin".
• Authoring step 6 and a new Authoring Checklist item reference the scope.
• Note under Integration Types that the Examples column is maintainer-managed.
• Note under Existing Plugin Conformance that the table/count is maintainer-managed.

Docs-only; no plugin or runtime behavior changes.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1