Rewrite CHANGELOG to match AGENTS section taxonomy

[Copilot]

This updates CHANGELOG.md to follow the section structure now required by AGENTS.md. Existing release notes were reclassified into the new subsection taxonomy, and the Upcoming section now contains a properly scoped entry for this PR.

• What changed

  • Converted top-level release headings from # to ## for consistency with the new layout
  • Reworked historical entries into the required sections:
    • ### 🚀 Enhancement
    • ### 🐛 Bug Fix
    • ### 📝 Documentation
    • ### 🔩 Dependency Updates
    • ### 🏠 Internal
  • Added an Upcoming documentation entry with the PR link format required by AGENTS.md

• Effect on changelog structure

  • Normalizes old entries that previously used mixed labels such as Features, Improvements, and Fixes
  • Makes future additions align with the repository’s current changelog rules without changing release content

• Example

  ## Upcoming
  
  ### 📝 Documentation
  
  Rewrote this changelog to use the AGENTS.md section layout. ([#246](https://github.com/dandi/s3-log-extraction/pull/246))

[CodyCBakerPhD]

@copilot These do not lead with - as items in a list, nor do they have the PR link URLs

[Copilot]

@copilot These do not lead with - as items in a list, nor do they have the PR link URLs

Fixed in 42083c9.

[CodyCBakerPhD]

Fixed in 42083c9.

@copilot That only did a few entries; please update all

[Copilot]

Fixed in 42083c9.

@copilot That only did a few entries; please update all

Fixed in 6e1ff21.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.88%. Comparing base (5130621) to head (6e1ff21).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #246      +/-   ##
==========================================
- Coverage   73.43%   71.88%   -1.55%     
==========================================
  Files          41       40       -1     
  Lines        1547     1494      -53     
==========================================
- Hits         1136     1074      -62     
- Misses        411      420       +9     

Flag	Coverage Δ
unittests	71.88% <ø> (-1.55%)	⬇️

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see 1 file with indirect coverage changes

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