Add the extension point mechanism

[pournasserian]

Closes #2438

What

The mechanism every extension point uses, landed before any domain interface exists (first slice of #2429).

• [ExtensionPoint] marks an interface in FluentCMS.Core.Extensions as a seam.
• IExtensionCatalog lists every registered extension with its contract, implementation and whether it is built in (FluentCMS.Core.Builtin).
• AddFluentCmsExtension<TContract, TImplementation>() registers explicitly; AddFluentCmsExtensions(assembly) discovers every concrete implementation of an extension-point interface. Both produce the same catalog entry and the same IEnumerable<TContract> resolution, and a discovered type already registered explicitly is not duplicated.
• AddFluentCmsCore() discovers the Core assembly's built-ins through the same call a plugin assembly would use, so there is no privileged path (EXT-02).
• CONTRIBUTING.md gains an "Extension points" section.

Why

Field types (F-SCH), check types (F-CHK), tool providers (F-MCP), storage, scorers and notification channels all need one registration and discovery story. Landing it first means each of those interfaces arrives with its first consumer on top of a mechanism that is already tested.

Tests

Explicit registration resolves and is catalogued; assembly discovery finds every implementation of an extension point and nothing else; discovery after an explicit registration does not duplicate; a contract without the attribute is rejected; Core provides a catalog with no built-ins yet.

Checklist

• Linked to an issue a maintainer has confirmed
• Targets v2
• Tests or docs updated where behaviour changed
• No tool-generated co-author trailers or footers