chore(skill): add menlo-persistence agentskills.io skill

[DigiBanks99]

Summary

Adds the menlo-persistence agentskills.io skill to skills/menlo-persistence/ so that AI-assisted agents can implement future EF Core + PostgreSQL persistence slices correctly and consistently, without re-deriving architectural conventions from scratch on each task.

Closes #243

What's in the skill

SKILL.md covers:

• Project layout: Menlo.Application is the sole EF Core project; domain model stays in Menlo.Lib
• Slice interface pattern: feature handlers inject IUserContext / IBudgetContext etc., never MenloDbContext directly
• Entity type config conventions: ToTable with schema, numeric(18,4) for monetary values, automatic snake_case via EFCore.NamingConventions, no HasColumnName() calls
• Strongly typed IDs: uuid mapping via central value converters in ConfigureConventions
• ISoftDeletable: contract + SoftDeleteInterceptor + global query filter in OnModelCreating
• IAuditable: contract + AuditingInterceptor (fires on EntityState.Added / Modified)
• AddMenloApplication(): DI registration with interceptors + scoped slice interface registrations
• TestContainers-only testing policy (in-memory EF Core is prohibited)
• Migration commands with exact --project / --startup-project flags

references/patterns.md provides full working C# code examples for every convention: MenloDbContext skeleton, DI extension, both interceptors, slice interface, entity type config, TestContainers fixture, and example integration tests.

evals/evals.json contains 3 benchmark evals with 24 graded assertions:

1. BudgetEntry full persistence slice (entity + config + interface + DI + migration)
2. IUserContext integration tests (TestContainers, fresh scopes, soft-delete, audit)
3. HouseholdMember entity (schema placement, typed ID, ISoftDeletable, IHouseholdContext)

Benchmark results (iteration 1)

	With skill	Without skill	Delta
Pass rate	100%	66%	+34pp

Key failures in the without-skill baseline:

• Missing PostgreSQL schema in ToTable (e.g., "budget" schema omitted)
• Wrong decimal precision: numeric(18,2) instead of numeric(18,4)
• Manual HasColumnName() calls instead of relying on snake_case convention
• Repository pattern used instead of slice context interface
• EnsureCreated instead of MigrateAsync in test fixtures
• Wrong --project flag in migration commands

Acceptance criteria

• Skill accurately describes the slice interface pattern
• Skill covers both interceptor implementations (auditing + soft delete)
• Skill includes the TestContainers-only testing policy
• Skill references this repository for concrete examples (via references/patterns.md)
• Benchmark demonstrates meaningful improvement: 100% vs 66% (+34pp)
• At least one subsequent AFK slice is successfully implemented using only the skill (validated via feat(persistence): Menlo.Application infrastructure + Aspire PostgreSQL wiring #244-feat(persistence): Transparent soft deletes via SoftDeleteInterceptor #247)