documentation site: plan, milestone and sync skills

Milestone "Documentation site", the agents half. Siblings: platform !1 (merged) (merged), docs docs!2 (merged).

Issues: #6 (closed) plan records dependencies as references and phase labels; #7 (closed) milestone orders by the Depends on references; #8 (closed) plan scaffolds the milestone narrative; #9 (closed) sync script; #10 (closed) sync skill and the Workspace instructions; #12 (closed) Python package, GitLab client and pytest in CI; #13 (closed) plan in Python with tests; #14 (closed) sync and order in Python with tests. #11 (closed) (the backfill) was a run against GitLab, recorded and closed in the issue.

The shell scripts of #6 (closed) to #10 (closed) were replaced by the corvus Python package before merge (uv, click, python-gitlab, PyYAML; pytest in the dev group; 28 tests, red then green per behaviour). The commands are uv run corvus-plan, uv run corvus-sync run <docs> and uv run corvus-order from the agents checkout, on the glab session's token. This repo gains its first pipeline: the tests on MRs and main, plus the references lint.

Findings that changed the plan: GitLab Free has no blocking issue links, so the order of work lives in the Depends on section of each issue as references, with relates_to links as a mirror. ADR 0003 is amended in the docs MR.

Nothing left to the operator here. Merge this one before docs (docs' pipeline includes ci/ from agents main).