docs(structural): convert patterns.md to thin index

[mghabin]

Converts patterns/patterns.md from a doctrine document into a thin index.

Each named pattern now gets a ≤3-sentence summary plus a required link to the owning chapter section. Decision-bearing prose lives in the chapters.

Ownership mapping applied

• Outbox → docs/03-data.md §6 (Cross-system consistency → Outbox)
• CQRS → docs/03-data.md §12 (CQRS-lite)
• Repository / Specification → docs/03-data.md §11
• Unit of Work = DbContext → docs/03-data.md §6
• ProblemDetails / IExceptionHandler → docs/02-aspnetcore.md §3
• Resilience pipeline → docs/02-aspnetcore.md §7 + docs/06-cloud-native.md §6
• Caching matrix → docs/03-data.md §14 + docs/02-aspnetcore.md §9 (OutputCache)
• WebApplicationFactory → docs/04-testing.md §5
• TimeProvider / FakeTimeProvider → docs/01-foundations.md §5 (prod) + docs/04-testing.md §14 (test)
• Health checks → docs/06-cloud-native.md §10
• Aspire ServiceDefaults → docs/06-cloud-native.md §1
• BackgroundService + Channel → docs/02-aspnetcore.md §12
• DI keyed services → docs/01-foundations.md §5

Kept here as cross-cutting (no single chapter owner)

• Vertical-slice vs Clean Architecture (architectural axis choice)
• Bounded contexts / monorepo strategy (links to monorepo.md)
• Domain events vs integration events (failure-mode contrast)

Anti-patterns section continues to point at patterns/anti-patterns.md.

GoF refresher reduced to a one-line-per-pattern vocabulary cheat-sheet pointing at the BCL primitive that replaces the hand-roll.

Markdownlint passes locally with the repo's .markdownlint.yaml.