What

This PR adds an agent skill to the FluentAssertions NuGet package. The skill teaches AI coding agents (Claude Code, GitHub Copilot, Codex, ...) how to write good assertions with Fluent Assertions. It works the same way as in Mockly.

• SKILL.md (repository root) is based on the documentation. It contains:
  • the main rules, such as "call Should() on the real subject" and "avoid Should().BeTrue()";
  • a rewrite table from weak to expressive assertions, taken from the tips pages;
  • a short reference for each kind of subject, all BeEquivalentTo options, exceptions (sync and async), events, type/architecture assertions, AssertionScope, global configuration, custom assertions and common pitfalls.
• The package includes the skill under skills/fluentassertions/, with a .gitignore.
• buildTransitive/FluentAssertions.targets copies both files to .agents/skills/fluentassertions/ in the root of the consuming repository before each build. It finds the root through SourceLink (the git root), the solution directory, or the nearest AGENTS.md. If it finds none, it skips the copy without an error.
• The copied .gitignore keeps the generated SKILL.md out of source control.
• Set <FluentAssertionsSkill>false</FluentAssertionsSkill> to turn it off. The introduction page explains this.

How I tested it

• dotnet pack puts skills/fluentassertions/SKILL.md, skills/fluentassertions/.gitignore and buildTransitive/FluentAssertions.targets in the package.
• I built a test project in a separate git repository that references the local package. The build copies both files, git ignores SKILL.md, and with FluentAssertionsSkill=false nothing is copied.
• I compiled the code samples from the skill against the package to make sure the API names and namespaces are correct.

🤖 Generated with Claude Code