re #15 docs: Courier package guide

[tonydspaniard]

Summary

Courier (command bus) package documentation, per #15. Single new file: docs/packages/courier.md (~493 lines). Follows the #16 template.

Covers Command/Handler/Bus/Locator/Resolver/middleware pipeline. Both ExecCommandBusConfiguration and MiddlewareCommandBusConfiguration are documented with their COURIER_MAP_FILE env var requirement.

Source observations

• The issue checklist asked about a "CommandResolver" — in Courier this role is split between name resolvers (ClassCommandMessageNameResolver, CommandMessageNameResolver) and locator services (InMemoryCommandLocatorService). There is no single "CommandResolver" class. Both layers documented.
• CommandLockerMiddleware queues re-entrant dispatches; inner commands run AFTER the outer handler completes. Test suite verifies this ordering. Documented.
• Async dispatch is not supported. Stated explicitly in Limitations.

Spun out as a separate issue

• courier: LogMessageTrait::withLogMessage() mutates $this instead of returning a clone #47 — LogMessageTrait::withLogMessage() mutates $this while named with* (convention violation vs. the framework's immutability idiom).

Test plan

• Render and skim
• Pure additive docs