Replace pr skill with local version and add issue skill

[kmcginnes]

Summary

Swap the vendored pr skill for my extended version, which adds a publish workflow, related-issue linking, and language screening, and add the matching issue skill.

 .agents/skills/
+├── issue/
+│   └── SKILL.md
 └── pr/
     ├── CREDITS.md
     ├── SKILL.md
-    ├── agents/openai.yaml
+    └── references/evidence.md

skills-lock.json now records both as local sources, not mattpocock/skills.

Merge Danger

Door: two-way

Impact: agent-only

Only changes agent tooling. A future upstream sync will no longer refresh pr from mattpocock/skills.

[kmcginnes]

Approved