Fix #369: add Unregister-AntigravityMcp to PowerShell uninstaller

[NyxSpecter4]

Fixes #369

The PowerShell uninstaller (scripts/uninstall-monk-agent.ps1) never removed the Antigravity MCP server registration that start-monk-agent.ps1 creates via Register-AntigravityMcp. After uninstall on Windows, a dead monk entry remained in ~/.gemini/config/mcp_config.json pointing at a binary that no longer exists. The POSIX uninstaller already calls remove_antigravity_mcp() — this restores platform parity.

What changed

• Added Unregister-AntigravityMcp to uninstall-monk-agent.ps1, mirroring Register-AntigravityMcp (same path logic, JSON parse/remove/write with temp-file swap).
• Called before the final uninstall complete message.
• Removes the monk server entry; drops the mcpServers key if it becomes empty.

Verification

• POSIX parity: uninstall-monk-agent.sh already calls remove_antigravity_mcp() (2 refs) — unchanged.
• Register-AntigravityMcp still idempotent on install; Unregister-AntigravityMcp is its inverse on uninstall.
• Script authored and reviewed against the live main source; cannot run Windows PowerShell CI here, but the JSON edit path is the same safe temp-swap pattern the launcher already uses.

Happy to adjust if the maintainers prefer the function live in a shared module.