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

.github/skills/medassist-observability-guard/SKILL.md

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+---
+name: medassist-observability-guard
+description: Ensure MedAssist changes preserve actionable logging, health checks, and clear operational error visibility, including equivalent requests phrased in German.
+---
+
+# Skill Instructions
+
+Use this skill when changes affect backend services, schedulers, integrations, startup flow, or failure handling.
+
+## Objective
+
+Maintain operational visibility so failures are detectable, diagnosable, and actionable.
+
+## Required Checks
+
+1. Critical paths keep clear error reporting.
+2. Health-check behavior remains intact and meaningful.
+3. Logs contain actionable context without leaking secrets.
+4. Errors are surfaced with enough detail for debugging.
+5. Silent failure paths are avoided.
+
+## MedAssist Focus Areas
+
+- `backend/src/index.ts`
+- `backend/src/routes/health.ts`
+- `backend/src/services/*`
+- Scheduler and notification flows
+
+## Anti-Patterns
+
+- Swallowed exceptions.
+- Generic logs with no context.
+- Missing visibility for background failures.
+
+## Response Format
+
+Return:
+
+- Observability touchpoints reviewed
+- Gaps found and suggested fixes
+- Operational risk level