Add diffsitter skill documentation

[qdrddr]

Add documentation for the diffsitter skill, including its purpose, supported languages, examples, CLI options, and practical workflows for Coding Agents such as Claude Code, Codex, Gemini etc.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.50%. Comparing base (3386289) to head (3f9ffaa).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1277   +/-   ##
=======================================
  Coverage   61.50%   61.50%           
=======================================
  Files          15       15           
  Lines        1356     1356           
=======================================
  Hits          834      834           
  Misses        522      522           

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[afnanenayet]

Is it typical to have these things hardcoded? Was wondering if it would be better to tell Claude to pull the supported languages by running the subcommand to list supported languages.

Also this can be built with support for dynamically loading tree sitter grammars so you're not actually restricted to the grammars I built with, so would mention that as well.

The CLI flags might also be worth pulling dynamically, or at least telling Claude to also spot check against the --help text.

[afnanenayet]

I'm not sure what the best practices are for things like this, I can also see the argument for not having Claude constantly parse this information