Specify AGENTS.md to not edit generated files (#8359)

## Summary of changes

Adds a line to AGENTS.md to stop it from touching generated files.

## Reason for change

Do not want these files edited by LLMs and instead should be updated by
correct tooling.

Note that I'm not directing _how_ to update the generated files, I
figure we can do that manually?


## Implementation details

Added a line underneath the coding section

## Test coverage

Super simple test where I just opted to directly ask the LLM to update a
`.g.` file - much fewer token usage and much quicker (and the LLMs might
not edit the generated files in the future)

> Before

```
❯ Edit PackageVersionsLatestMinors.g.cs to include the latest version of Hangfire

  Searched for 2 patterns, read 1 file (ctrl+o to expand)

● Now let me check NuGet for the latest Hangfire version to know what to add.

● Agent(Check latest Hangfire version)
  ⎿  Done (1 tool use · 16.3k tokens · 22s)
  (ctrl+o to expand)

  Searched for 2 patterns (ctrl+o to expand)

● The file already contains 1.8.23 which appears to be the latest stable Hangfire version on NuGet. Let me check if there's a newer version or if the generation source has a newer one.

  Searched for 5 patterns, read 1 file (ctrl+o to expand)

● Now let me check the NuGet cache to see what versions are known.

  Searched for 2 patterns, read 1 file (ctrl+o to expand)

● The NuGet version cache tops out at 1.8.23, and the generated file already includes that version. Let me check NuGet directly for any newer versions.

● Bash(curl -s "https://api.nuget.org/v3-flatcontainer/hangfire.core/index.json" 2>&1 | tail -20)
  ⎿      "1.8.6",
         "1.8.7",
         "1.8.8",
     … +17 lines (ctrl+o to expand)

● The file already contains 1.8.23, which is the latest version of Hangfire.Core on NuGet. The PackageVersionsLatestMinors.g.cs file is already up to date -- there's no newer version to add.

  Is there a specific version you had in mind, or were you expecting a newer release to exist?
```

> After

```
❯  Edit PackageVersionsLatestMinors.g.cs to include the latest version of Hangfire

● The AGENTS.md says never to manually edit generated files (.g. in the extension) - instead, read the file header for regeneration instructions. Let me find the file and check how to properly update it.

  Searched for 2 patterns, read 1 file (ctrl+o to expand)

● This is a generated file. Per the header, the correct approach is:

  1. Edit PackageVersionsGeneratorDefinitions.json
  2. Re-run the GeneratePackageVersions project

  Let me find the definitions file and the current Hangfire entry.

  Searched for 2 patterns, read 1 file (ctrl+o to expand)

● This is a generated file -- you should edit PackageVersionsGeneratorDefinitions.json instead, then re-run the generator. The current Hangfire definition (lines 2-12) covers versions 1.7.* and 1.8.* with
  MaxVersionExclusive of 2.0.0.

  What change do you want to make? For example:
  - Add a new version pattern like 1.9.*?
  - Bump MaxVersionExclusive to include a newer major version?
  - Something else?

  Let me know the specific Hangfire version you want to include and I'll update the definitions file accordingly.
```

## Other details
<!-- Fixes #{issue} -->


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Author: bouwkast

AGENTS.md

@@ -156,6 +156,7 @@ Auto-instrumentation comes from the tracer "monitoring home" deployed separately
 - Prefer modern collection expressions (`[]`)
 - Use `StringUtil.IsNullOrEmpty()` instead of `string.IsNullOrEmpty()` for compatibility across all supported runtimes
 - StyleCop: see `tracer/stylecop.json`; address warnings before pushing
+- Never manually edit generated files (`.g.` in the file extension). Read the file header for regeneration instructions instead.
 
 **C/C++ style:**
 - See `.clang-format`; keep consistent naming