Define the local model runtime abstraction #59

Problem

Local runtimes need the same session and event model as remote providers, which requires an explicit shared abstraction before implementation.

Scope

• Define the local runtime contract, session expectations, events, and capability reporting
• Keep local runtimes compatible with the shared provider and session model
• Clarify how local runtimes participate in mixed-agent sessions

Out of scope

• Choosing specific model files or benchmark targets
• UI polish beyond the contract needed for later implementation

Implementation notes

• Do not invent a separate local-only session model
• Keep the abstraction broad enough for LLamaSharp and ONNX Runtime
• Exclude MLXSharp from the first wave

Definition of Done

• The issue defines the local runtime abstraction clearly
• Later local-runtime issues can implement against one stable contract

Verification

• Review the issue against the domain model and session-composition issues

Dependencies

• Parent epic: [Epic] Add local runtime support beyond external provider CLIs #19
• Depends on: [Epic] Establish dotPilot as a desktop control plane for local-first agent operations #11