build(ci): run the docs-integrity gate on Node 24

[bketelsen]

Summary

The docs-gate job in .github/workflows/ci.yml still provisioned Node 20,
which reached end-of-life on 2026-04-30. This moves it to Node 24, the
current Active LTS, and updates the two places that state the Node minimum
so the docs no longer disagree with CI.

Exactly three lines change:

1. .github/workflows/ci.yml:242 — node-version: "20" -> "24". The
   actions/setup-node SHA pin (820762786026740c76f36085b0efc47a31fe5020,
   v7.0.0) is unchanged, and no other job or workflow line is touched.
2. AGENTS.md:58 — the tooling bullet now reads Node 24+, noting the 20.x
   end-of-life date and that 24 is the Active LTS.
3. scripts/check-docs.mjs:6 — header comment Node >= 20 -> Node >= 24.

No script logic, .coverage-thresholds.json value, threshold, Makefile
recipe, or gate is changed or weakened, and no npm ecosystem was added to
.github/dependabot.yml.

Next target: Node 26 becomes Active LTS on 2026-10-28. A reviewer who
would rather wait and jump straight to 26 can close this — 24 is simply the
LTS that is current today.

Checks

• make check — gofmt leaves no diff, golangci-lint run
  (.golangci.yml) clean, go test -v ./... green (unit + tests/e2e)
• go test -race -short ./... green
• go.mod still declares no dependencies (stdlib only)
• New or changed behavior has focused tests, including failure paths;
  exact output changes update docs/specs/reporter-package.md
  — n/a, no Go behavior changes

Docs housekeeping

• AGENTS.md, docs/design/overview.md, docs/specs/* updated for
  behavior or convention changes — AGENTS.md tooling bullet
• Conformance aliases (ADR-0001) untouched — canonical targets edited
  instead

Protected boundaries

• Changes under .github/workflows/** are called out above as high
  risk. This is a supply-chain change: it raises the runner's Node
  major only. No action is added, no SHA pin is altered, and job
  permissions: stay contents: read.

Verification

• node scripts/check-docs.mjs green
• Checked against the
  PR review rubric

node --version on this branch

v24.19.0

node scripts/check-docs.mjs

ok   docs_index_coverage: 1.000 (required 1)
ok   link_integrity: 1.000 (required 1)
ok   symlink_resolution: 1.000 (required 1)
checked: 12 docs, 153 links, 9 symlinks
exit=0

make verify

==> verify: go.mod is tidy
go mod tidy -diff
==> verify: gofmt
==> verify: golangci-lint 2.13.1 (built with go >= 1.26.7)
0 issues.
0 issues.
go vet ./...
go vet ./_examples/deploy ./_examples/fileprocess ./_examples/healthcheck ./_examples/migration 
==> verify: tests
go test ./...
ok  	github.com/frostyard/std/reporter	0.002s
ok  	github.com/frostyard/std/tests/e2e	3.043s
exit=0

No Node 20 references remain

$ grep -rn 'Node 20\|node-version\|Node >= 20' --include='*.md' --include='*.yml' --include='*.mjs' .
.github/workflows/ci.yml:242:          node-version: "24"