chore: clean up import hygiene in src/agentics/core

[mnlscn]

chore: clean up import hygiene across src/agentics/core

Summary

• Removes 16 unused imports (F401) across the core package — dead symbols left over from commented-out code, refactors, and copy-paste
• Removes 6 duplicate/redefined imports (F811) — symbols imported more than once at module level
• Hoists 28 mid-file imports (E402) to the top of their respective modules — most were scattered inside function blocks or after load_dotenv() calls with no technical reason to be deferred
• Adds __all__ = ["AG"] to core/__init__.py to make the AG re-export explicit

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Files changed

File	What changed
core/__init__.py	Added __all__ = ["AG"] to mark AG as an intentional re-export
core/agentics.py	Removed unused Callable (typing) and PydanticTransducerVLLM (superseded)
core/default_types.py	Removed unused asdict, is_dataclass, Any, Generic, create_model
core/mellea_pydantic_transducer.py	Removed unused asyncio, Optional, RejectionSamplingStrategy; hoisted tqdm/mellea/pydantic imports while preserving load order (tqdm must be patched before mellea loads)
core/semantic_operators.py	Removed unused pathlib and Field
core/transducible_functions.py	Hoisted 9 mid-file stdlib/pydantic/agentics imports to the top block; consolidated two separate utils imports into one
core/utils.py	Hoisted 10 mid-file imports (math, json, jsonfinder, ast, textwrap, types, hashlib, etc.) to the top; removed unused numerize and duplicate create_model
core/vector_store.py	Removed redundant mid-file re-import of BaseModel, ConfigDict, Any, Dict, List (already at top)

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Notes

• mellea_pydantic_transducer.py: import order for tqdm / mellea is intentionally kept below
  os.environ["TQDM_DISABLE"] = "1" and the tqdm.tqdm = no_bar patch.
  These imports carry # noqa: E402 with an explanatory comment — this is a genuine load-order constraint, not a circular import.

• No functional changes — only import structure.