Align CI Node.js and pnpm runtime versions

[kylymo]

Parent issue

Part of #885 — Update app's GitHub Apps and automation

Objective

Align the repository's Node.js and pnpm versions across the package manifest, CI, README, and agent guidance.

Evidence

• package.json declares Node ^24.x and pnpm@10.20.0.
• .github/workflows/ci.yml currently uses Node 22 and pnpm latest.
• Repository documentation contains conflicting runtime guidance.

Scope

• Align .github/workflows/ci.yml, package.json, README.md, and AGENTS.md.
• Pin CI to the chosen Node major and pnpm version.
• Consider adding .node-version or .nvmrc.
• Verify a frozen-lockfile install, lint, typecheck, and tests.

Acceptance criteria

• One tested Node/pnpm matrix is documented and used by CI.
• CI no longer installs pnpm latest.
• Fresh checkout instructions are reproducible.
• Verification commands pass.

Out of scope

Dependency upgrades unrelated to runtime alignment and application source changes.