docs: amend constitution to v2.1.0 (.NET 10/C#14, coverage gates, NuGet currency)

[hindermath]

Summary:

• amend constitution to v2.1.0
• require .NET 10 and C# 14.0 toolchain baseline
• enforce CI coverage gate (>=70%) with target >=80%
• require NuGet dependency currency and documented pinning exceptions
• propagate changes across templates, command templates, and runtime guidance docs

Validation:

• constitution sync report updated
• no unresolved constitution placeholders
• governance version/date fields updated