docs: add 'when to use' triggers + examples to all patterns
Added 'When to Use' subsections with concrete decision triggers and
before/after Go code examples to patterns across all directories:
- patterns/error-handling.md (3 patterns: sentinels, wrapping, Join)
- patterns/concurrency.md (4 patterns: Mutex, Once, done channels, pipelines)
- patterns/interfaces.md (4 patterns: small interfaces, accept/return, adapter, optional)
- patterns/structs.md (3 patterns: zero-value, constructors, config structs)
- patterns/package-design.md (3 patterns: internal/, init(), context keys)
- patterns/style.md (3 patterns: interface checks, iota constants, named types)
- patterns/testing-advanced.md (3 patterns: table tests, golden files, httptest)
- patterns/api-conventions.md (3 patterns: Must, layered API, graceful shutdown)
- patterns/documentation.md (2 patterns: examples, deprecated)
- kubernetes/patterns.md (3 patterns: controller, workqueue, leader election)
- kubernetes/production-go.md (2 patterns: codegen, HandleCrash)
- smells/anti-patterns.md (2 anti-patterns: cache mutation, edge-triggered)
aweiker
eb9171368b