Improve prpack CLI import safety

[rogerchappel]

What changed

• Guarded the CLI entrypoint so importing dist/src/cli.js no longer runs prpack generate as a side effect.
• Added a regression test that imports the built CLI module, confirms run is exported, and verifies no PR_PACK.md is created.

Why it matters

Prpack exports its CLI runner for automation and tests. Import-time side effects can surprise toolchains, create files in the caller's working directory, and make programmatic use less trustworthy.

Verification

• npm ci && npm run check && npm test && npm run smoke && npm run release:check — passed.

Remaining limitations / follow-up

• The version command is still hard-coded to 0.1.0; a future PR could read it from package metadata like the library export path does.