Backfill changelog for v0.12.0-v0.12.5

[hmgaudecker]

Summary

• CHANGES_template.md stopped at v0.11 even though six releases have shipped since
  (v0.12.0 through the just-tagged v0.12.5).
• Reuses the content for v0.12.0-v0.12.4 from the previously-closed Backfill changelog for v0.12.0–v0.12.4 #214, cross-checked
  against the actual git log for each tag range to confirm accuracy (each bullet maps
  to a real commit; routine dependency-bump PRs are omitted, consistent with how the
  rest of the changelog already treats them).
• Adds a new v0.12.5 entry summarizing Auto-install npm deps for pixi run pytask; sync config with boilerplate #221 (the pixi run pytask npm-install fix and
  boilerplate sync merged as the v0.12.5 tag).
• Does not include Ignore miktex.org in the docs link checker #222 (the miktex.org link-check ignore), since it landed after the
  v0.12.5 tag and isn't part of a release yet — it'll get picked up whenever the next
  version is tagged.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.34%. Comparing base (3e3956c) to head (c3b76f3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #223   +/-   ##
=======================================
  Coverage   99.34%   99.34%           
=======================================
  Files          10       10           
  Lines         153      153           
=======================================
  Hits          152      152           
  Misses          1        1           

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