docs: extend CONTRIBUTING.md with code guidelines

[jlaportebot]

Summary

This PR extends CONTRIBUTING.md with a comprehensive section on code guidelines and conventions, addressing issue #4023.

Changes

Added a new "Code Guidelines" section that documents:

1. Naming Conventions

   • File names ({service}_{api}.go pattern)
   • Receiver names (consistently use 's')
   • Request option types
   • Request body types
   • Response types
   • Method and variable naming
   • Common variable names (owner, repo, org, user, team, project)
   • Enterprise and organization scoped methods
   • Common structs (ListOptions, ListCursorOptions, UploadOptions, Response)

2. JSON Tags for Request Bodies

   • Required fields: non-pointer types without omitempty
   • Optional fields: pointer types with omitempty
   • Use omitzero for structs and slices

3. URL Tags for Query Parameters

   • All fields should be non-pointer types with url tags
   • Use omitempty to omit zero values

4. Pagination

   • Offset-based pagination (ListOptions)
   • Cursor-based pagination (ListCursorOptions)
   • Pagination best practices (embedding options, handling nil, using iterators)

5. Common Types

   • ID fields: always int64
   • Node ID fields: always string
   • Timestamp fields: use Timestamp type
   • Boolean fields: always *bool

6. Generation Patterns

   • Generated accessors
   • Generated iterators
   • Generated documentation
   • Linter rules (sliceofpointers, structfield)

Benefits

• Helps new contributors understand the codebase better
• Reduces review burden by making expectations explicit
• Improves code consistency across the project
• Documents patterns that are frequently discussed in reviews

Testing

This is a documentation-only change. No code changes or tests needed.

Closes #4023

[gmlewis]

Thank you, @jlaportebot.
LGTM.

cc: @alexandear

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.71%. Comparing base (6c58592) to head (6fcb261).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4202   +/-   ##
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  Coverage   93.71%   93.71%           
=======================================
  Files         209      209           
  Lines       19772    19772           
=======================================
  Hits        18529    18529           
  Misses       1046     1046           
  Partials      197      197           

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