fix: register Antigravity MCP on native Windows

[Napikha1818]

Summary

• register Monk in Antigravity's global MCP config from native Windows setup
• preserve unrelated config and existing MCP server properties
• create absent/null/non-object mcpServers and refresh stale Monk endpoint URLs
• leave malformed or non-object root JSON untouched with a warning
• run registration on both successful launcher paths
• test all three rendered PowerShell launchers on Windows CI

Test plan

• powershell.exe -NoProfile -ExecutionPolicy Bypass -File tests/register-antigravity-mcp-windows.ps1
• powershell.exe -NoProfile -ExecutionPolicy Bypass -File tests/block-monk-windows.ps1
• PowerShell AST parsing across every .ps1
• byte parity across all three rendered PowerShell launchers
• POSIX sh -n across tracked shell scripts
• git diff --check

Fixes #148

[Napikha1818]

PR #149 is ready for review.

Validated on Windows PowerShell 5.1:

• released v0.1.47: focused registration test RED
• PR head: focused registration test PASS
• stale endpoint reconciliation
• malformed/non-object config byte-preserved
• deep unrelated config preserved
• all three rendered launchers byte-identical
• PowerShell AST and existing Windows hook tests pass

GitHub currently shows no check runs and mergeable_state=unstable. Could a maintainer enable the workflow when convenient?