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

.github/skills/medassist-db-compat-check/SKILL.md

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+---
+name: medassist-db-compat-check
+description: Enforce backward-compatible database changes for MedAssist SQLite and Drizzle migrations, including equivalent requests phrased in German.
+---
+
+# Skill Instructions
+
+Use this skill for any feature or fix that adds or reads persisted data.
+
+## Mandatory Sequence
+
+For every new persisted field/column:
+
+1. Add the column in `backend/src/db/schema.ts` with `NOT NULL DEFAULT <value>`.
+2. Generate migration with Drizzle Kit.
+3. Add matching `ALTER TABLE` logic in `backend/src/db/client.ts` inside `runAlterMigrations()`.
+4. Read values null-safe in routes/services (`?? defaultValue`).
+
+## Hard Rules
+
+- Never remove or rename existing columns.
+- Never add non-null columns without defaults.
+- Never read newly added fields without fallback.
+- Never manually edit generated Drizzle SQL migrations.
+
+## Verification Checklist
+
+- Schema update exists.
+- Generated migration exists.
+- Alter migration for existing DBs exists.
+- Runtime reads are fallback-safe.
+
+## Response Format
+
+Report these items explicitly:
+
+- New/changed columns
+- Added alter-migration statements
+- Null-safe read locations
+- Remaining migration risk (if any)