Node.js migration + dev toolchain cleanup

[lex00]

Part of the Op / Temporal deployment workflow redesign.

Chant currently runs on Bun. Migrating to Node.js unblocks use of the Temporal TypeScript SDK worker package, which requires authentic Node.js.

Tasks

• Replace Bun-specific APIs with Node.js equivalents across packages/core
• Update package.json scripts to use node/tsx instead of bun
• Update bin/chant entry point
• Verify test suite passes under Node.js (458 tests, 0 failures)
• Update CI workflow to run on Node.js
• Update packages/test-utils for Node.js compatibility
• Update root justfile recipes
• Replace all remaining bun/bunx references in scripts, Dockerfiles, docs, and examples
• Fix workspace smoke test (207 passed, 0 failed):
  • Slurm lexicon: require() → createRequire in training-job.ts (ESM fix)
  • Slurm lexicon: correct initTemplates() structure, add chant-slurm primary skill, fix skill file names
  • Docker lexicon: compose.ts recognized as valid init source file
  • Dockerfile.smoke: generate slurm dist/manifest.json inside container