refactor: one node/pm bootstrap story (mise vs setup-node)

[jleni]

Problem

Three patterns coexist:

1. mise-action@v4 — _checks-ts, expo, rs, r2, pulumi, gen_proto, …
2. actions/setup-node@v4 (+ sometimes bun) — _publish-npm, _ledger_main
3. Prebaked zondax/ubuntu-ci images with node/pnpm/bun already installed

Not broken, but harder to reason about than a single house style. Also does not use zondax/actions/setup-node-env / setup-mise yet.

Proposal

Design + migrate toward one preferred path:

• Prefer mise (or zondax/actions/setup-mise / setup-node-env) for non-image jobs
• Document when the ubuntu-ci image is enough without reinstalling toolchains
• Avoid dual mise + setup-node in the same job unless required

Acceptance

• Written convention in README
• _publish-npm / ledger either justified dual-setup or simplified