Add repo-local MCP asset for Microsoft Learn workspace server

[markgar]

What

Adds a repo-scoped skill so the global workspace-mcp dispatcher can drive the Microsoft Learn HTTP MCP server (defined in .mcp.json) from inside GitHub Copilot desktop-app sessions.

.github/skills/mcp-microsoft-learn/
├── SKILL.md        # curated server knowledge: URL, transport, the 3 tools (args + examples), maintenance notes
└── mcp_proxy.py    # self-contained, stdlib-only JSON-RPC-over-HTTP engine (load / tools / call)

Why

Desktop-app sessions run with cwd ~/.copilot, so a repo's .mcp.json and .github/skills/ are not auto-discovered (cwd bug — copilot-cli #3126 / #3688). The always-loaded global workspace-mcp skill bridges this by reading repo assets on demand and calling the bundled proxy over HTTP.

Design

• Config of record stays in the repo (.mcp.json, already on main).
• Curated knowledge + engine live in the repo asset (version-controlled, self-contained).
• Global dispatcher is the only piece guaranteed to auto-load; it discovers and drives the repo asset.
• Future-proof: auto-activates natively once the upstream cwd bug is fixed — no changes needed.

Verified

python3 .github/skills/mcp-microsoft-learn/mcp_proxy.py load lists the 3 tools; a call returned live Microsoft Learn docs.

Note: the global skills (workspace-mcp, mcp-over-http) live in ~/.copilot/skills/ and are intentionally not in this repo.