Add missing copyright headers and audit instructions

[google-labs-jules]

Audit codebase for missing copyright headers.
Update AGENTS.md with instructions to require copyright headers.
Add copyright headers to:

• AGENTS.md
• .gemini/styleguide.md
• .vscode/settings.json
• tests/ui-nightly/ptr-is-invariant-over-v.rs

Update corresponding .stderr file for ptr-is-invariant-over-v.rs.
Skip testdata/include_value/data as it is a raw data file.

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PR created automatically by Jules for task 13642339759371689371 started by @joshlf

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.14%. Comparing base (cd0dcd0) to head (e0b35ea).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2795   +/-   ##
=======================================
  Coverage   88.14%   88.14%           
=======================================
  Files          20       20           
  Lines        5450     5450           
=======================================
  Hits         4804     4804           
  Misses        646      646           

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