Align CI tooling with runtime targets (Node >=22 / Bun) #2

Problem

The shared CI workflow uses pnpm and Node 18/20/22, while packages declare runtime targets of Node >=22 and Bun >=1.3. This can hide runtime-specific issues and diverge from the supported toolchain.

Evidence

• .github/workflows/ci.yml uses pnpm and a Node matrix including 18.x/20.x.
• Package manifests in omniscript-core/converters/site declare Node >=22 and Bun >=1.3.

Expected

• Align CI to the supported runtime (Node 22+ and/or Bun).
• If legacy Node versions are intentionally supported, update package engines/docs accordingly.

Acceptance Criteria

• CI matrix matches declared engine support.
• Build/test steps use the intended package manager (Bun or pnpm) consistently.