node.verify.yml: add opt-in format-check step

[gingur-bot]

Context

gingur/devkit's reusable workflow .github/workflows/node.verify.yml runs three consumer scripts: lint, typecheck, test. The fleet is adopting oxfmt for formatting; verify should be able to enforce oxfmt --check. Devkit is consumed via @main, so any unconditional new step would instantly break consumers that lack the script — the step must be opt-in via input.

Changes

In .github/workflows/node.verify.yml:

• Add input format (camelCase single word per repo conventions): description "pnpm script to run for format check. Empty skips the step.", type string, default ''.
• Add a Format step between Lint and Typecheck: if: inputs.format != '', run: pnpm ${{ inputs.format }} (same pattern as the other steps, honoring working-directory).

In README.md: update the verify recipe to show format: fmt:check and note it's opt-in.

Verification (evidence required in the PR)

• Workflow syntax validated: act run of a throwaway caller (or actionlint) showing (a) step skipped when format omitted, (b) step runs pnpm fmt:check when set.

Acceptance criteria

• Input + guarded step added exactly as above; omitting the input produces byte-identical behavior to today; README recipe updated; validation evidence cited in PR.