feat: add copilot skill wrapper

Copilot agent and chat on github.com will ignore SKILL.md
in .claude/skill.
Add thin wrapper with references.

Author: afranken

.github/copilot-instructions.md

@@ -0,0 +1,13 @@
+# Copilot Instructions for S3Mock
+
+Read `AGENTS.md` (root + relevant module) before making any changes — it contains architecture,
+DO/DON'T rules, code style, and conventions that apply to all tasks.
+
+## Task-Specific Guidance
+
+Detailed instructions for each task type are maintained in `.claude/skills/`:
+
+- **Implementing features / fixing bugs**: [`.claude/skills/implement/SKILL.md`](../.claude/skills/implement/SKILL.md)
+- **Writing / updating tests**: [`.claude/skills/test/SKILL.md`](../.claude/skills/test/SKILL.md)
+- **Refactoring**: [`.claude/skills/refactor/SKILL.md`](../.claude/skills/refactor/SKILL.md)
+- **Updating documentation**: [`.claude/skills/document/SKILL.md`](../.claude/skills/document/SKILL.md)