docs: consolidate copilot governance and add medassist skills (#160)

.github/skills/medassist-release-handoff/SKILL.md

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+---
+name: medassist-release-handoff
+description: Enforce MedAssist release ownership by preventing remote git/release actions by normal agents and delegating to release-manager, including equivalent requests phrased in German.
+---
+
+# Skill Instructions
+
+Use this skill when a request includes branch push, PR creation, merge, tagging, release notes publishing, or release orchestration.
+
+## Ownership Rules
+
+- Remote git/release actions are owned by `@release-manager`.
+- Normal agent/Copilot must not perform:
+  - `git push`
+  - PR creation/merge
+  - tag/release creation
+
+## Required Behavior
+
+1. Perform local code edits only.
+2. Summarize local changes clearly.
+3. Provide handoff instruction to `@release-manager` for shipping steps.
+
+## Response Format
+
+When this skill applies, return:
+
+- "Release handoff required"
+- Delegate target: `@release-manager`
+- Shipping checklist (branch, PR, CI, merge, release)