major: upgrade to node24

[kwilcox]

Summary

• Bumps action runtime from node20 to node24 (action.yml), .nvmrc to v24.14.0, and @types/node to ^24.0.0.
• Bumps CI workflow Node version to 24 and pinned GitHub Actions to versions that themselves run on Node 24: actions/checkout@v6, actions/setup-node@v6, cycjimmy/semantic-release-action@v6.
• Ports upstream nick-fields/retry#165 (16b9199) and #166 (5b7030b).

No src/ or tsconfig.json changes are needed — upstream didn't change either, and the existing TypeScript compiles cleanly under Node 24. dist/index.js is byte-identical after rebuilding with ncc against the new @types/node@24, so it isn't included in the diff.

Test plan

• npm install succeeds and produces a clean lockfile (adds undici-types@7.18.2 as a transitive devDep of @types/node@24).
• npm test passes locally on Node 24 (v24.15.0).
• ncc build src/index.ts produces a clean bundle.
• CI passes (unit + integration jobs) on Node 24.