chore: document type system conventions + enforce ABC/Protocol rules (AC-492)

[jayscambler]

Summary

Documents the type system conventions that emerged from the cleanup sweep and adds contract tests to enforce them going forward.

Changes

CONTRIBUTING.md — new "Type System Conventions" section

Codifies the conventions already established by PRs #576-589:

• ABC vs Protocol — ABC for internal hierarchies with shared implementation, Protocol for duck-typed external integration points
• Dict types — dict[str, Any] as sole convention (not dict[str, object]), prefer TypedDict for known shapes
• Collection params — Sequence[X] for read-only list params, Mapping[str, X] for read-only dict params
• Type aliases — LlmFn, StrEnum import style, logger naming

test_interface_conventions.py — 2 contract tests

1. test_protocols_do_not_use_abstractmethod — Protocol classes shouldn't have @abstractmethod (it's redundant for structural typing)
2. test_abc_classes_have_abstractmethod — ABC classes must have at least one @abstractmethod (otherwise use Protocol)

Both tests pass on current code — the convention is already followed.

Verification

• ruff check src — all checks passed
• mypy src — zero errors
• pytest tests/ — all tests pass (including 2 new convention tests)

Issue

Resolves AC-492

[linear]

AC-492 Standardize ABC vs Protocol usage — 12 ABCs and 5 Protocols with inconsistent boundaries