feat(acp): Add GitHub Copilot ACP provider

[matdev83]

feat(acp): Add GitHub Copilot ACP provider

Related: https://docs.openclaw.ai/tools/acp-agents

Current architecture / implementation constraints

Implement this as a thin external ACP connector on top of the current shared ACP runtime:

• Reuse connector-support/acp as the single ACP protocol/session/tool/stream implementation (refactor: deduplicate ACP runtime #242).
• Package Copilot integration under connectors/ through the executable backend-plugin/process-host architecture (feat: add executable backend connector plugins #208); do not add a root built-in ACP implementation.
• Declare execution_class: agent_runtime; direct routing remains supported while feat(routing): agent-runtime execution composition safety #337 rejects unsafe weighted/parallel/thinker/failover composition by default.
• Keep provider-specific code limited to GitHub Copilot launch/auth/configuration/discovery/lifecycle behavior that cannot be handled by the shared ACP support layer.
• Preserve current cancellation, structured error propagation, process supervision, secret handling, and optional dependency isolation.
• Prefer connector-local contract/parity/process-tree tests and generic ACP TCK coverage over provider-specific core branches.

No dedicated Kiro SDD or implementation PR exists yet.