ADR 0020: Split built-in agent backends and extract a shared agent runner

Date: 2026-01-14

Context: planning and coding duplicated the same retry loop, task construction, truncation, and debug-dir logic, while concrete CLI backends and core contracts/utilities were not clearly separated. Decision: Create a dedicated core package loreley.core.worker.agent for shared contracts and orchestration utilities (AgentTask, AgentInvocation, run_agent_task, resolve_worker_debug_dir, TruncationMixin, plus loader/utils) and implement built-in CLI backends under loreley.core.worker.agent.backends.* (CodexCliBackend, CursorCliBackend). Constraints: Keep loreley.core.worker.agent backend-agnostic (no imports/re-exports of built-in backends) so imports remain acyclic; built-in backends may depend only on loreley.core.worker.agent APIs. Consequences: Worker agents become thinner and easier to evolve; adding new backends requires less boilerplate, and configuration strings should reference loreley.core.worker.agent.backends.