fix(ci): drop node 20 matrix and align engines with pi-coding-agent peer

[code-yeongyu]

Summary

Main CI is red on the node 20 matrix (both OSes) since the @earendil-works peer migration: @earendil-works/pi-coding-agent declares engines.node >=22.19.0 and ships undici 8.5.0, which throws webidl.util.markAsUncloneable is not a function on node 20 (see the failing run on main tip 9a2aaa8). pi-rules' declared >=20.0.0 engine range is no longer real.

• package.json engines: >=20.0.0 → >=22.19.0 (match the peer floor)
• CI matrix: ["20", "22"] → ["22", "24"]

The main-branch ruleset's required status checks need the matching context rename (node 20 → node 24); done alongside this PR.

Test plan

• Full suite passes locally (252 tests) on node 26.
• CI on this PR runs the new matrix.

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Summary by cubic

Align Node.js support with @earendil-works/pi-coding-agent by setting engines.node to >=22.19.0 and updating CI to test Node 22 and 24. This removes Node 20 from the matrix and fixes CI failures caused by undici 8.5.0 on Node 20.

• CI matrix: ["22", "24"]

• package.json engines: >=22.19.0

• Migration

  • Use Node >=22.19.0 locally and in deployment.
  • Update required status checks from “node 20” to “node 24” if your branch rules depend on them.

^{Written for commit 18509b9. Summary will update on new commits.}