Create client parity compatibility matrix document #35

Context

Part of #10: Copilot client parity across surfaces.

Problem

As we test Squad on VS Code, JetBrains, and GitHub.com, we need a single source of truth documenting what works where.

Deliverable

Create a compatibility matrix document showing:

Include:

• Row for each feature/capability
• Column for each client surface
• ✅ (works), ❌ (not available), ? (untested), ⚠️ (limited/degraded)
• Notes on parameter differences, fallbacks, or workarounds
• Link to detailed investigation issues (Feature: Enable MCP use #11, Add @copilot coding agent as a Squad member type #12, Add @copilot coding agent as a Squad member type #13)

Location

Place in docs/features/client-parity.md or docs/scenarios/client-compatibility.md

Success Criteria

• Compatibility matrix created with all tested surfaces
• CLI column is 100% verified
• VS Code column populated from Feature: Enable MCP use #11, Add @copilot coding agent as a Squad member type #12, Add @copilot coding agent as a Squad member type #13 results
• Documentation includes fallback strategies
• Added to main docs navigation

Related

• Parent: Copilot client parity: Squad works fully only on GitHub Copilot CLI #10 (Copilot client parity)
• Depends on: Feature: Enable MCP use #11, Add @copilot coding agent as a Squad member type #12, Add @copilot coding agent as a Squad member type #13 (VS Code investigations)