refactor: remove dead DeclaredContract/TypeConstraint infrastructure

[tob-scott-a]

DeclaredContract and TypeConstraint were defined in models/annotations.py and exported from models/init.py, but the data they carried was pure redundancy with existing fields on CodeUnit:

• TypeConstraint(parameter_name, declared_type) == Parameter.name + Parameter.type_ref.name, both already on CodeUnit.
• DeclaredContract.return_constraint == CodeUnit.return_type.name.
• DeclaredContract.parameter_types iterates Parameter entries we already have via CodeUnit.parameters.

The 14 tree-sitter parsers all called build_contract(params, return_type) and bound the result to contract, then used it purely as a boolean in if contract is not None: graph.annotations.setdefault(func_id, []). setdefault with an empty list adds nothing observable (preanalysis later prunes empty annotation lists), so the entire pipeline was dead.

Zero consumers in QueryEngine, zero CLI surface, zero skill references. Decision: kill it. If type-level queries become necessary later, they can read Parameter.type_ref and CodeUnit.return_type directly, or a dedicated QueryEngine method can be added then with real semantics.

Removed:

• models.DeclaredContract, models.TypeConstraint (deleted from annotations.py and the models/init.py exports/all).
• _common.build_contract (helper and its imports).
• 14 per-parser contract = build_contract(...) calls and the dead if contract is not None: ... setdefault(...) blocks.
• The TestContractBuilding test class; the useful sub-assertions (return_type capture, parameter count) moved to TestTypeCapture.

842 tests pass, lint/format/type check clean.