chore: drop node 20, add node 26 to CI matrix, bump .nvmrc to 24

[ava-silver]

Summary

• Removes Node 20 from the CI test matrix (min supported is now Node 22)
• Bumps `.nvmrc` from 18 → 24 to match the CI default

Test plan

• CI passes on Node 22 and 24

[ava-silver]

• chore: drop node 20, add node 26 to CI matrix, bump .nvmrc to 24 #701 👈 (View in Graphite)
• main

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.69%. Comparing base (ec7793a) to head (5fdeac9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #701   +/-   ##
=======================================
  Coverage   77.69%   77.69%           
=======================================
  Files          12       12           
  Lines        1112     1112           
  Branches      350      350           
=======================================
  Hits          864      864           
  Misses        118      118           
  Partials      130      130           

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