chore(repo): pin Node at the root, and drop a Sponsor button that leads nowhere

[beyondnetPeru]

Closes #565, closes #563.

#565 — nothing enforced the Node version

CI runs Node 20 and CONTRIBUTING said to pin it locally, but there was no .nvmrc and no root engines, so a contributor on 18 or 24 found out from a failing job.

• .nvmrc with 20 — read by nvm, fnm, asdf and volta alike
• root engines: { "node": ">=20 <21" }, the range the workflows actually use
• .nvmrc added to the root-cleanliness allowlist. The guard rejected it first, exactly as the issue said it would
• the taxonomy's root policy updated in both languages to match

It warns, it does not block. engine-strict is off, verified on this machine's Node 24: npm install emits EBADENGINE and exits 0. The published CLI keeps engines.node >= 18.0.0 — that is a statement about consumers, not about building this repository, and CONTRIBUTING now says so in both languages.

Three stale claims corrected in the same taxonomy section, since the change had to edit it anyway: it still listed DOCUMENTATION_VERSIONS.md (no such file), rulesets/ at the repository root (it moved under src/ months ago), and asserted "No application src/ directories are maintained in this repository" — now the opposite of true.

#563 — the Sponsor button offered nothing

The issue reported four keys with no value. Two of them did have values, so the file was checked rather than trusted:

Key	State
github: [beyondnetcode]	hasSponsorsListing: false — the button led to a page that does not exist
sponsor: [beyondnetcode]	not a key GitHub's FUNDING.yml schema defines
patreon:	empty
open_collective:	empty

Nothing in the file was real, so the file is gone — which is what the issue proposed for that case.

Verification

• 03-validate-root-cleanliness.mjs passes with .nvmrc present: 46 root entries read, 3 ignored by git, 43 checked against the taxonomy
• guards 01 and 04 green, 17/17 entry-surface documents (CONTRIBUTING is on that surface, so both halves moved together)

🤖 Generated with Claude Code

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[github-actions]

📊 Bilingual Coverage Impact

PR Changes

• Paired EN/ES files modified: 3
• New EN files needing ES translation: 0

Repository Coverage

Metric	Value
Total EN files	525
Total ES files	495
Paired files	0
Coverage	0%

✅ Good: All EN changes have ES counterparts.

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[beyondnetPeru]

08-validate-tracking.mjs was right to go red, and the failure was mine.

GT-555 closed on six evidence artifacts and one of them was .github/FUNDING.yml, which this PR deletes. A closed gap pointing at a file that no longer exists is a closure nobody can re-check, so the guard refused — correctly.

Fixed in b5998c60, without repointing or quietly dropping anything:

• the artifact leaves GT-555's evidence list, and the record states in its own words why, so the removal does not read as an omission
• the acceptance criterion is kept in both catalog halves and annotated as superseded, because it was satisfied when it was written — rendering was never the problem, what it rendered was
• the other five artifacts are untouched and still carry the closure

08-validate-tracking.mjs passes locally.