feat: Add AGENTS.md

[jamescrosswell]

Closes #4971

Summary

Adds AGENTS.md at the repo root (with a CLAUDE.md symlink) per the repository-docs standard and the agents-md skill. Also added a symlink for copilot as that's what I'm using mostly in Rider.

Testing

Tested by asking (in copilot in my case, but using the Claude Sonnet 4.6 model): Implement https://github.com/getsentry/sentry-dotnet/issues/4105 and create a pull request for this. Claude implemented this in:

• feat: add repro guide and needs-repro automation #4975

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Semver Impact of This PR

⚪ None (no version bump detected)

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📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

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[jamescrosswell]

@Flash0ver I think this is probably a good starting point. We can always tweak it later after we've used it a bit if we find there are things it's consistently doing or not doing properly.

[Flash0ver]

question: kinds of breaking changes

Binary incompatible / Source incompatible / Behavioral change

Shall we clarify that we are generally fine with Binary Breaking Changes in minor versions,
but not with Source Breaking changes ... and that we consider Behavioral Breaking changes only in rare exceptions?

[Flash0ver]

for future consideration ... could also be in CONTRIBUTING.md
we also do evaluate on a case-by-case basis

[Flash0ver]

question: Directory.Build.props and Directory.Build.targets in sub-directories?

I noticed quite often that AI coding tools don't get MSBuild's hierarchical structure correctly:

1. first Directory.Build.props/.targets found upwards from the project's location
2. more parent Directory.Build.props/.targets files only if imported explicitly

So I'm wondering, if we should mention the other files, too ... something like:

- `samples/Directory.Build.props` / `samples/Directory.Build.targets` - shared MSBuild properties across all projects in the `samples` directory
- `src/Directory.Build.props` / `src/Directory.Build.targets` - shared MSBuild properties across all projects in the `src` directory
- `test/Directory.Build.props` / `test/Directory.Build.targets` - shared MSBuild properties across all projects in the `test` directory

[Flash0ver]

for future consideration

[Flash0ver]

question: .editorconfig

I wonder if it's worth pointing out to our .editorconfig for our C# code conventions.
Maybe in a separate section ## C# 🤔

[Flash0ver]

future consideration: run dotnet format ... before every commit (considering the .editorconfig)