Prepare atomcommit release candidate surface

[rogerchappel]

Summary

• Tightens the README around the current atomcommit CLI behavior, read-only git diff inputs, fixture smoke workflow, and V0.1 limitations.
• Updates changelog and roadmap entries so the release-candidate surface describes the implemented planner instead of generic setup.
• Strengthens scripts/smoke.sh with help/version checks plus fixture-backed Markdown and JSON plan assertions.

Verification

• npm run release:check
  • 11 node tests passed
  • npm run smoke passed against the mixed-change fixture
  • npm pack --dry-run produced atomcommit-0.1.0.tgz with 10 files and the expected CLI/runtime/docs surface

Release Candidate Notes

• Package is closest to a V0.1.0 CLI release after maintainer review.
• Recommended GitHub topics: git, cli, atomic-commits, agent-tools, developer-workflow.
• No publish, tag, or release was performed.

Risk Level

• Low. Documentation and smoke-check only; no runtime planner behavior changed.

Rollback Plan

• Revert commit 50b67b7 if the README or smoke assertions are not the desired release-candidate direction.