docs: trim CI section in README, link to actions guide

[theoephraim]

Summary

• The CI / GitHub Actions section in the README had grown to ~130 lines of YAML that duplicated docs/github-actions.md and was drifting out of sync (e.g. README showed the old single-job release workflow while the docs had moved to the split-job ci plan + --expect-mode pattern).
• Replace it with a short summary of the three bumpy ci subcommands (check, plan, release) and a pointer to the actions guide for ready-to-copy workflows, token setup, and trusted publishing.
• Promote "Local versioning and publishing" to its own h2 since it's no longer nested under the CI section.
• Drive-by polish: split the old "Pack-then-publish" feature bullet into separate "OIDC + staged publishing" and "Custom release targets" lines, add a "No additional action/app needed" feature bullet, and surface bumpy ci setup in Getting Started.

Net change: 159 lines → 20 lines in README.md.

Test plan

• Read through the rendered README on the PR and confirm the CI section reads cleanly
• Confirm the GitHub Actions setup guide link resolves
• Spot-check that no other docs/links still reference the removed README anchors