✨ Add skills for EF migrations and new ADR creation

Introduces new skills for managing Entity Framework Core migrations
and creating Architectural Decision Records (ADRs). These skills
provide structured guidance for users to document architectural
decisions and manage database migrations effectively.

Co-authored-by: Copilot <copilot@github.com>

.github/skills/ef-migration/SKILL.md

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+---
+name: ef-migration
+description: 'Add, remove, or apply Entity Framework Core migrations against a project under src/. Use when: creating a new migration, reverting the last migration, generating a migration bundle, applying migrations to a database.'
+argument-hint: 'Describe the schema change (e.g., "add audit columns to Orders") and the target DbContext if multiple exist'
+---
+
+# EF Core Migration
+
+## Prerequisites
+
+- The EF project (containing the `DbContext`) is referenced by — or is itself — a runnable startup project.
+- Requires `dotnet ef`: `dotnet tool restore` (if a manifest exists) or `dotnet tool install -g dotnet-ef`.
+
+## Conventions
+
+- **Migration name**: `PascalCase`, imperative, third person, ≤ 60 chars (e.g., `AddsOrderAuditColumns`, not `audit_changes` or `Migration1`).
+- **Output folder**: default `Migrations/` inside the EF project. Do not relocate.
+- **One DbContext per command** — pass `--context` whenever the project has more than one.
+- **Never edit a migration after it has been applied to a shared environment.** Add a corrective migration instead.
+- **Commit migrations together with the model change** that produced them.
+
+## Procedure
+
+The **EF project** contains the `DbContext`; the **startup project** is the runnable project EF executes against (usually a web/worker referencing the EF project).
+
+1. **Add the migration:**
+   ```bash
+   dotnet ef migrations add <PascalCaseName> \
+     --project src/<EFProject> \
+     --startup-project src/<StartupProject> \
+     --context <DbContextName>
+   ```
+2. **Inspect** the generated `*.cs` and `*.Designer.cs` files for unintended changes (unrelated columns, dropped indices).
+3. **If wrong, remove before committing:**
+   ```bash
+   dotnet ef migrations remove --project src/<EFProject> --startup-project src/<StartupProject>
+   ```
+
+For a deployable migration binary, use the [docker-build](../docker-build/SKILL.md) skill's `migrations` target.
+
+## Generating an SQL Script
+
+```bash
+dotnet ef migrations script <FromMigration> <ToMigration> \
+  --project src/<EFProject> \
+  --startup-project src/<StartupProject> \
+  --idempotent \
+  --output artifacts/migrations.sql
+```
+
+Use `--idempotent` whenever the script may run against a database in an unknown state.
+
+## Troubleshooting
+
+- **"Unable to create a 'DbContext'…"** — pass `--startup-project` explicitly; auto-detection picks the wrong project in multi-project solutions.
+- **Multiple DbContexts found** — pass `--context <Name>`.