chore(toolchain): Node.js 24 + pnpm 12.14.0 migration #29

Part of umbrella firstsun-dev/.github#38 (Node.js 24 + pnpm 12.14.0 toolchain standardization).

Scope

Node.js 24 for CI/build, exact pnpm@12.14.0 pin (packageManager), frozen-lockfile installs, pnpm 12 config migration (pnpm-workspace.yaml, allowBuilds, overrides), CI/Docker/release consistency. Preserve published runtime compatibility, release provenance and tests. No deploys, no unrelated dependency upgrades.

Dockerfile currently uses Node 22 and prepares pnpm 10.33.0.

Acceptance

• Clean + pnpm install --frozen-lockfile with pnpm 12.14.0
• lint / typecheck / tests / build pass
• CI + Docker/release paths verified
• PR merged, then close this issue