Isolate runtime queues and public contract imports (TD-01, TD-05) #286

Problem

Constructing default runtime state loads the engine and concrete Loop nodes through engine-owned queue definitions. Importing public protocols also registers Code and LLM nodes through implementation imports, so importing contracts changes executable node availability.

Requested behavior

• Move ready queue contracts, task values, and the default implementation under runtime while retaining existing engine import identities, custom queue injection, and snapshot formats.
• Separate the code executor contract from its implementation and leave Code/LLM registration to explicit class or implementation imports.
• Use Import Linter to enforce general architectural layers across all top-level modules through ordinary local hooks and CI. Reject upward dependencies, including type-only imports, and require new top-level modules to be assigned to a layer.
• Document the explicit bootstrap requirement and complete TD-01 and TD-05 in the technical debt register.

Context

This follows the architecture review in #275 and builds on #285 (laipz8200/knowledge-review-metadata). The implementation is on laipz8200/runtime-import-boundaries.