Fix Composer discovery for all public classes

[chubes4]

Summary

• extend the package-owned lazy Composer classmap from package contracts to the complete public src/ type surface
• keep procedural registration and runtime bootstrap owned by agents-api.php
• prove conversation, run-outcome, runtime-tool, transcript, and tool-executor contracts resolve in a consumer-only PHPStan fixture

Verification

• consumer-only PHPStan: passed with no custom config or stubs
• Agents API max-level PHPStan: passed
• Composer autoload outside WordPress remains inert and public classes remain lazy
• normal bootstrap: passed
• version-skew bootstrap: passed
• complete source vocabulary/bootstrap assertions: passed

Closes #526