[Epic] Integrate external providers and let sessions choose one or many agents

[KSemenenko]

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