Add GitHub Copilot SDK as an ADE agent provider

[linear-code]

Context

Investigate and implement GitHub Copilot SDK as a first-class ADE coding-agent provider. The SDK is GA and exposes the production Copilot CLI runtime over JSON-RPC, which is structurally similar to ADE's existing app-server and managed-agent integrations.

Potentially relevant surfaces include session creation/resume, streaming events and history, tool permissions, MCP, models, usage/compaction, hooks, and custom-agent/subagent lifecycle. Child output can be associated through parentToolCallId.

General guidance

• Do fresh research against the current TypeScript SDK and ADE provider abstractions before choosing the adapter shape.
• Confirm authentication, Copilot subscription/BYOK behavior, CLI lifecycle, and redistribution requirements.
• Normalize events into ADE's existing chat and subagent model; declare unsupported capabilities accurately.
• Keep the integration isolated behind the provider boundary and add targeted coverage for lifecycle, approvals, resume, and nested agent events.

Starting sources

• https://github.com/github/copilot-sdk
• https://docs.github.com/en/copilot/how-tos/copilot-sdk/troubleshooting/compatibility
• https://docs.github.com/en/copilot/how-tos/copilot-sdk/features/streaming-events
• https://docs.github.com/en/copilot/how-tos/copilot-sdk/features/custom-agents
• https://docs.github.com/en/copilot/how-tos/copilot-sdk/features/fleet-mode

[linear-code]

ADE-120