docs: adds AGENTS.md to guide coding agents

[rosygmiki]

This PR creates an AGENTS.md file that will serve as a starting point for all AI models.

It references files such as CONTRIBUTING.md and file-format.md, and goes through what an agent can and can not do within the repo.

Assisted-by: Claude Sonnet 4

[gemini-code-assist]

Code Review

This pull request introduces AGENTS.md, a new documentation file providing guidance and permissions for AI agents working on the tar-diff repository. The review feedback suggests expanding the list of core components requiring prior approval to include patching logic and clarifying that compiled binaries should not be committed to the source repository.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.19%. Comparing base (e78a156) to head (b91ac31).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #97   +/-   ##
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  Coverage   79.19%   79.19%           
=======================================
  Files          10       10           
  Lines        1115     1115           
=======================================
  Hits          883      883           
  Misses        134      134           
  Partials       98       98           

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[knecasov]

I added some comments.

[djach7]

Left one comment but otherwise LGTM to after other comments from Klara are addressed.

One additional super duper nit: I think there's a space missing in the commit message, right after the colon. I think it should be docs: add AGENTS.md

[knecasov]

LGTM, thank you very much for incorporating the comments!

[djach7]

Thank you for updating, LGTM!