[SDK-6d] Extension point: IToolWrapperContribution

[ChrisonSimtian]

Expose IToolWrapperContribution as a public extension point for third-party tool wrappers. Maps to surface 4 in RFC #2.

Scope

• Plugins register a new *Tasks static facade.
• The 62 first-party wrappers in Fallout.Common.Tools.* continue to ship in-box but become the reference shape.
• Pairs with Split tool wrappers out of Fallout.Common into per-tool projects + NuGet packages #72 (per-tool packages) — once a contribution model exists, the in-box wrappers become a candidate to extract.

Depends on

SDK-1.