feat(ci): skip heavy test suites for docs-only changes

[midleman]

Summary

Skip the heavy test suites (unit, ext-host, e2e) for docs-only changes, on both the PR and merge-to-branch workflows.

• New scripts/is-docs-only-change.sh classifies a diff as docs-only (Markdown anywhere, docs/**, root LICENSE*/CHANGELOG*/NOTICE*); any other file forces full CI.
• PR workflow keeps the three required jobs running but skips their steps when docs-only, preserving the required check names.
• Merge workflow skips the test jobs outright (not branch-protected).
• @ :full-ci in the PR body forces full CI as a one-way override.
• Zero-install bash unit test for the detector, wired into the pr-tags job.

QA Notes

Detection fails safe to full CI on any error or empty diff. This PR is itself a code change, so it correctly runs full CI; the docs-only skip can only be exercised by a docs-only PR once this lands on main - see example here.

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical

^{readme  valid tags}