feat(skills): ship ue-design-skills bundle, licensing and delivery gate

Phase 0 of the handoff plan, as a marketplace rather than a flat skills/
directory. Content moved out of the LyraResearch archive and depersonalised:
addresses stay in the archive, recipes ship.

- plugins/ue-design-skills: 17 skills, 232 failure-mode entries, each with the
  six required fields; catalog.json as the harness-neutral source of truth and
  .claude-plugin/ as one adapter over it.
- _gate: 16 rules, one poisoned fixture per rule, plus surface coverage so a
  declared file cannot silently miss the line rules.
- ADR-0002 (harness-neutral bundle behind a marketplace) and ADR-0003 (split
  licensing: CC BY-ND 4.0 prose, Apache-2.0 code and metadata).
- LICENSE files at both levels, CONTRIBUTING.md, docs/licensing-options.md as
  the material the licence decision grew from.

Verified: gate.py 0 violations; test_gate.py 16/16 rules redden on their
fixtures with a clean baseline and 2 root files reaching the line rules.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

plugins/ue-design-skills/_gate/fixtures/clean/skills/sample-skill/SKILL.md

@@ -0,0 +1,25 @@
+---
+name: sample-skill
+description: >-
+  Gate fixture. A minimal clean skill bundle that proves the delivery gate
+  accepts valid content, including a donor name inside a Provenance section.
+---
+
+# Sample skill
+
+> A reference project shows you a shape, not a contract.
+
+## 1. Norm
+
+State the field, name its consumer, and give a recipe that reproduces the check
+in the reader's own tree. A norm that cannot be re-derived is decoration.
+
+## Provenance
+
+Measured on Epic's Lyra Starter Game for Unreal Engine 5.6. Source addresses
+stay in the research archive; the recipes below reproduce each finding in your
+own project. This section is the only place the donor may be named.
+
+## 2. Failure modes
+
+Detection recipes and guardrails: [failure modes](references/failure-modes.md).