Add cookbook with real-world usage patterns

[danielstarman]

Summary

Add a cookbook section (either in the README or a separate docs/cookbook.md) showing concrete, copy-pasteable patterns for real-world usage.

Motivation

The current README demonstrates the API with a toy Database -> Repo -> Service example. This is fine for understanding the API, but doesn't show why you'd reach for a DI container instead of just passing arguments manually. A cookbook with realistic examples would lower adoption friction and answer the "why miniject?" question.

Suggested patterns to cover

• CLI app startup - composition root that wires up config + services, passes to CLI handler
• Test fixtures - using scope() to override specific dependencies without rebuilding the graph
• Diamond dependencies - 15+ services where manual wiring gets painful, showing the real value proposition
• Optional dependencies - Database | None = None pattern for services with optional features
• Per-context scopes - different configurations for different execution contexts (e.g., batch vs interactive)
• Factory functions - when to use factories vs auto-wiring vs instances

Also consider

A short "Gotchas" section covering:

• from __future__ import annotations behavior with get_type_hints
• Forward reference timing
• Override scope (keyword overrides apply to the immediate constructor only, not the full graph)
• Why Annotated[...] is rejected and what to do instead