Unify deprecation policy: enforce 3 minor release minimum window

[SkalskiP]

Summary

• Documented a formal deprecation policy (minimum 3 minor releases before removal) in both CONTRIBUTING.md and AGENTS.md.
• Unified all 0.29.0 deprecations to use remove_in="0.32.0" — previously these ranged inconsistently from 0.31.0 to 0.34.0.
• docs/changelog.md — Added a full ### 0.29.0 <small>Jun 15, 2026</small> section with 23 entries covering all Added/Changed/Fixed/Deprecated items from the release, replacing the old "UnReleased" content that belonged to this version. Updated date_modified frontmatter to today.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81%. Comparing base (3d07099) to head (99aba8a).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #2324   +/-   ##
=======================================
  Coverage       81%     81%           
=======================================
  Files           66      66           
  Lines         8974    8974           
=======================================
  Hits          7225    7225           
  Misses        1749    1749           

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