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

.github/skills/medassist-security-sanity/SKILL.md

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+---
+name: medassist-security-sanity
+description: Apply baseline security checks to MedAssist code changes, especially for backend routes, auth flows, and input handling, including equivalent requests phrased in German.
+---
+
+# Skill Instructions
+
+Use this skill when a change touches backend routes, auth/session logic, file handling, imports/exports, or external input.
+
+## Objective
+
+Prevent common security regressions with fast, practical checks during implementation.
+
+## Required Checks
+
+1. Validate and sanitize external input at API boundaries.
+2. Enforce auth/authz server-side for protected actions.
+3. Ensure secrets/tokens are never hardcoded or logged.
+4. Avoid information leakage in error responses.
+5. Keep permission-sensitive operations explicit and auditable.
+
+## MedAssist Focus Areas
+
+- Route handlers in `backend/src/routes/`.
+- Auth-related code in `backend/src/plugins/` and auth routes.
+- Data import/export and sharing endpoints.
+- File/image upload and serving paths.
+
+## Anti-Patterns
+
+- Trusting frontend-only checks.
+- Accepting unchecked query/body/path input.
+- Returning raw internal errors to clients.
+- Weak defaults for sensitive operations.
+
+## Response Format
+
+Report:
+
+- Security-sensitive files reviewed
+- Findings by severity (critical/major/minor)
+- Concrete remediation actions
+- Residual risk (if any)