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

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+---
+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).

plugins/ue-design-skills/_gate/fixtures/clean/skills/sample-skill/references/failure-modes.md

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+# Failure modes: sample skill
+
+Gate fixture. One complete entry, in the exact shape the gate accepts.
+
+An entry is valid only if a reader who has never seen the donor project can run
+`Detect` in their own tree and get an answer. The three middle fields are the
+ones that cannot be written by someone who did not do the work.
+
+---
+
+### SS-01 - Editable property with no writer
+
+**Mechanism.** A tunable float is compared against a timestamp field, but the
+timestamp is never assigned anywhere in the codebase.
+
+**Why it is silent.** Time since an unset stamp is time since world start, which
+always exceeds the threshold. The comparison is permanently true, so the gate it
+was meant to impose never rejects anything and no error is ever raised.
+
+**Why the obvious check misses it.** The field is read twice and participates in
+a comparison, so any "is this used?" search answers yes. What is absent is the
+**writer**, not the reader.
+
+**Symptom.** A designer tunes the property for days and reports that it changes
+nothing.
+
+**Detect.** For each editable property, search for an assignment rather than a
+mention, and subtract the declaration -- a member declared with an initializer
+matches every assignment pattern you can write:
+
+    rg -n "\bPropertyName\b\s*(=|\+=|-=)" Source/ \
+      | rg -v "\b(bool|u?int\d+|float|double|F[A-Z]\w+)\s+PropertyName\b"
+
+If reads exist and this comes back empty, the property is a stub.
+
+**Guardrail.** Do not ship an editor-exposed property whose value has no writer.
+Every tunable needs one owning assignment site and one test that moves it.