Add coreex-aggregate skill (DDD domain objects)

[chullybun]

Summary

Adds the coreex-aggregate skill for adding/modifying DDD domain objects (aggregate roots, child entities, value objects) built on CoreEx.DomainDriven, per the L1 skills inventory in #151.

Intentionally domain-agnostic — unlike sample-anchored skills (coreex-adapter, coreex-subscriber), this skill uses generic {Aggregate}/{ChildEntity}/{ValueObject} placeholders throughout rather than referencing samples/src/Contoso.Shopping.Domain/ directly, per explicit direction for this skill.

What's included

• .github/skills/coreex-aggregate/SKILL.md — entry point: when to use / not use, quick reference, key references (instructions files + actual CoreEx.DomainDriven source, not samples)
• .github/skills/coreex-aggregate/references/workflow.md — full workflow:
  • Phase 0 — confirm a Domain layer is actually warranted (skip for CRUD-oriented domains)
  • Phase 1 — interview to branch into aggregate root / entity / value object
  • Step 1 — Aggregate Root pattern (Aggregate<TId,TSelf>, CreateNew/CreateFrom, OnCheckCanMutate/OnMutate)
  • Step 2 — Child Entity pattern (Entity<TId,TSelf>, internal mutation methods, consumer-authored Clone(PersistenceState) helper)
  • Step 3 — Value Object pattern (sealed record, invariants in init accessors)
  • Unit Testing Aggregates — new section generalizing samples/tests/Contoso.Shopping.Test.Unit/Domains/BasketTests.cs: aggregates have no injected dependencies (beyond reference data), making them the best unit-test target in the codebase for both happy-path and business-rule-rejection coverage
  • Guard Helpers Reference, PersistenceState Reference (with actual transition nuances — New stays New on Modify, Remove is terminal + read-only), Integration Events (not domain events) rationale, Guardrails
• .github/prompts/coreex-aggregate.prompt.md — /coreex-aggregate prompt bridge
• Wired into src/CoreEx.Template/CoreEx.Template.csproj

Key rules captured

• Constructor always private; CreateNew(...)/CreateFrom(...) are the only public construction paths
• Modify(...)/Remove(...) are the only mutation paths — Remove(...) is terminal (sets Removed + MakeReadOnly(), no restore path)
• Child-entity mutation methods are internal, never public
• OnCheckCanMutate() guard failures throw the exception carried by the failed Result (typically BusinessException); other mutation methods may instead return a failed Result directly (e.g. not-found) — tests need to assert each correctly
• No async I/O in domain classes — ever
• No native domain-event dispatch (MediatR-style) — only coarse integration events via Aggregate<TId,TSelf>.Events, forwarded by the Application layer
• Aggregate unit tests (WithGenericTester<EntryPoint> + Test.Scoped(...)) cover mutation logic that request-validator tests structurally cannot reach

Validation

• Self-reviewed via rubber-duck sub-agent twice (once for the base skill, once for the added testing section) — each pass caught real accuracy issues before commit (child-entity factory visibility, ThrowOnError() overstatement, PersistenceState transition nuances, Clone() mislabeling, a nonexistent Runtime.Clock API reference, an overbroad "always throws" claim, an overstated validator claim, and overly narrow reference-data wiring guidance) — all fixed
• dotnet build src/CoreEx.Template/CoreEx.Template.csproj -c Release — 0 warnings, 0 errors

Co-authored-by: Copilot App 223556219+Copilot@users.noreply.github.com

[Copilot]

Pull request overview

Adds a new coreex-aggregate skill to guide developers in creating/modifying DDD domain objects (aggregate roots, child entities, value objects) using CoreEx.DomainDriven, and wires the skill assets into the template packaging so they ship with generated .github/ AI assets.

Changes:

• Added .github/skills/coreex-aggregate/ skill entrypoint plus a detailed workflow reference (including aggregate unit testing guidance).
• Added a /coreex-aggregate prompt bridge under .github/prompts/.
• Updated src/CoreEx.Template/CoreEx.Template.csproj to copy/pack the new prompt + skill files.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
src/CoreEx.Template/CoreEx.Template.csproj	Includes the new coreex-aggregate prompt/skill files in the AI content pack and copy steps.
.github/skills/coreex-aggregate/SKILL.md	Introduces the coreex-aggregate skill (scope, quick reference, and links to authoritative references).
.github/skills/coreex-aggregate/references/workflow.md	Provides the end-to-end workflow for aggregates/entities/value objects and aggregate unit testing guidance.
.github/prompts/coreex-aggregate.prompt.md	Adds the agent-mode prompt wrapper to invoke the skill consistently.