Classmap the complete public Agents API type surface

[chubes4]

Problem

#525 made public package contracts Composer-discoverable, but consumers still cannot statically resolve the guarded global conversation, runtime-tool, transcript, workflow, and tool interfaces/classes under src/.

A Data Machine conversation-result convergence now surfaces dozens of consumer PHPStan class.notFound findings for real public Agents API contracts such as:

• WP_Agent_Conversation_Loop
• WP_Agent_Run_Outcome
• WP_Agent_Runtime_Tool_Request
• WP_Agent_Conversation_Completion_Policy
• WP_Agent_Transcript_Persister
• WP_Agent_Tool_Executor

Expected

Extend the package-owned lazy Composer classmap to the complete public class/interface/trait surface under src/, while retaining agents-api.php as the idempotent runtime and registration bootstrap.

Constraints

• Composer autoload outside WordPress remains inert.
• Public classes remain lazy; loading vendor/autoload.php must not define them eagerly.
• Procedural registration files remain owned by agents-api.php and must not execute through classmap generation.
• Normal and version-skew bootstrap behavior remains unchanged.
• Add a consumer-only PHPStan fixture covering conversation/runtime contracts with no custom config or stubs.