Rewrite registration functions + migrate all task registrations

[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

Rewrite register_task to split registration across both registries: domain functions go to the HandlerRegistry, build hooks (preprocess, postprocess, validate) go to the BuildHooksRegistry. Update all register_task(...) call sites across the codebase to use the new API with explicit string keys. Remove key: ClassVar[str] from all config dataclasses.

Acceptance criteria

• register_task(key, config_type, ...) writes domain functions to HandlerRegistry and build hooks to BuildHooksRegistry
• All existing register_task call sites migrated (hadrons, milc, contract, grid)
• key: ClassVar[str] removed from all config dataclasses
• All existing tests continue to pass
• No regressions in task registration or lookup

Blocked by

• Blocked by Build Hooks Registry + tests #2 (Build Hooks Registry)
• Blocked by Handler Registry rewrite + Protocols + tests #3 (Handler Registry)
• Blocked by Revise build_config to use Build Hooks Registry + tests #4 (build_config revision)

User stories addressed

• User story 1
• User story 12

[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.