Type ReasoningClientProvider.get as AgentLLMClient | None

`core/agent_harness/ports.py` declares `ReasoningClientProvider.get() -> Any | None`, but every implementation returns an agent LLM client (see `core/agent_harness/turns/default_reasoning_client.py`). **Change**: retype the Protocol return as `AgentLLMClient | None` (import from `core.llm.types`; if that import would create a cycle, stop and comment on this issue instead of adding a local alias). Fix call-site fallout. **Verify**: `make typecheck` clean; `make check-imports` still passes (no new cycle). --- **Ground rules for this change** - New or changed Protocol methods use a docstring-only body — no `...`, no `pass`, no `raise NotImplementedError` (AGENTS.md, Code Style). Do not convert pre-existing `NotImplementedError` stubs elsewhere — out of scope. - Remove any retired name or alias completely in the same PR — no compatibility re-exports. - CI's typecheck does not cover `tests/`: run `uv run mypy ` yourself. - No new module-level mutable state; dependencies arrive via constructor or parameter. - Before opening the PR follow CI.md (lint, format, typecheck, tests) and fill the PR template, including the AI-usage disclosure.