Eliminate register_a2a + update a2a module to use builder directly

[Michael628]

Target branch: feature/issue-1-registry-refactor (based off develop)
All PRs for this work should target this branch, not main.

Parent PRD

#1

What to build

Delete pyfm/a2a/register.py and the register_a2a calls in pyfm/a2a/__init__.py. The a2a module uses build_config and the BuildHooksRegistry directly — it has no domain handler functions to register. Update scripts/contract_a2a_diagrams.py to call build_config(ContractConfig, params, file_params) without passing a handler.

Acceptance criteria

• pyfm/a2a/register.py deleted
• register_a2a calls removed from pyfm/a2a/__init__.py
• scripts/contract_a2a_diagrams.py uses build_config without handler/get_handler parameter
• a2a config building works end-to-end using only the builder + build hooks registry

Blocked by

• Blocked by Revise build_config to use Build Hooks Registry + tests #4 (build_config revision)

User stories addressed

• User story 10
• User story 14

[Michael628]

Note: As of 98d105c, HandlerRegistry and BuildHooksRegistry classes have been replaced with module-level functions. registry.py → task_registry.py, hooks.py → build_hooks.py. References to class methods like HandlerRegistry.register() should now read task_registry.register(), and BuildHooksRegistry.register() → build_hooks.register(). _CALLABLE_FIELDS is now a ClassVar.