[Epic] Integrate external providers and let sessions choose one or many agents #15
Description
Problem
The product needs a tracked epic for the actual provider adapters and for session composition across one or many agents.
Scope
- Track SDK-first adapters for Codex, Claude Code, and GitHub Copilot
- Track session composition, capability visibility, resume, branch, and effective instructions
Out of scope
- Toolchain installation and auth UX
- Local runtime implementation beyond how local runtimes participate in session composition
Planned child issues
- Add a Codex provider adapter using ManagedCode.CodexSharpSDK
- Add a Claude Code provider adapter using ManagedCode.ClaudeCodeSharpSDK
- Add a GitHub Copilot provider adapter using GitHub.Copilot.SDK
- Build the provider capability matrix and effective instruction view
- Implement the session composer for one or many agents
Definition of Done
- Provider adapter issues exist and reference this epic
- The epic makes mixed-agent sessions an explicit product capability
Verification
- Child issues reference this epic
- The epic stays focused on provider integration and session composition