Give a work-in-progress plan a public link before the pull request merges

Context

The story behind this plan — what prompted the work and why it matters now.

Two publishing routes already exist in this repo and neither covers mid-work sharing to a public site. artifact-tools:plan-artifact records an artifact-url: in the plan spec and republishes the same claude.ai page across sessions, which is the right tool whenever a claude.ai link is acceptable. plan-agent:setup-sites scaffolds GitHub Pages so everything under docs/ deploys, and .github/workflows/deploy-pages.yml publishes on every push to main touching docs/**. The gap sits between them: Pages only deploys from main, so a plan on a feature branch has no public URL until the pull request merges. This skill closes that gap for the case where the audience cannot use a claude.ai link — an external stakeholder, a public roadmap, anyone who needs a plain github.io address.

Three decisions shaped the design and are settled, not open. Previews live at docs/previews/<slug>.html, outside the plans directory. The gallery builder walks only the plans directory — kit/plugins/plan-agent/hooks/build-index.sh:87-88 is the os.walk(plans_dir) call and the exclusion list applied to it — so a preview outside that directory cannot produce a duplicate gallery card and needs no change to that hook; .github/workflows/regen-plans.yml filters on docs/plans/**, so it never fires either. Publishing to the plan's real path on main was rejected outright: the branch and main would both carry edited copies of the same generated HTML, guaranteeing a merge conflict on a file nobody should hand-resolve. Previews stay link-only — they are not listed in any gallery, because a public index of stale mid-work previews is worse than no index. The write uses the GitHub Contents API, not a local checkout of main — a gh api -X PUT needs no second working copy, survives a dirty working tree, and its sha precondition makes two sessions publishing the same plan fail loudly instead of silently clobbering one another.

Known risks, each with its mitigation. Writing to main outside the pull-request flow skips review and CI, so the skill asks for confirmation on every single run and every write it makes to main lands under docs/previews/ — never source, never the plan's real path. A repository that never enabled Pages would otherwise take that write and never deploy it, leaving an orphan commit on main, so the preflight confirms Pages is on and sends the user to plan-agent:setup-sites before anything is written. A repository with branch protection rejects the write, so a 403 or 409 is reported plainly with artifact-tools:plan-artifact offered as the fallback rather than being worked around. Previews would otherwise accumulate on main forever, so the skill offers to remove one as soon as the plan's status: reads completed.