fix: detect Antigravity Monk MCP structurally

[GioLealhmx]

Summary

• Detect existing Antigravity Monk MCP registration by parsing .mcpServers.monk instead of grepping the whole JSON file.
• Preserve unrelated config values that happen to equal "monk".
• Keep the three POSIX launcher copies in sync.
• Add a regression test covering unrelated "monk" text, registration, preservation of an existing server, and second-run idempotence.

Root cause and impact

The launcher skipped Antigravity MCP registration whenever "monk" appeared anywhere in mcp_config.json. An unrelated preference or description could therefore suppress .mcpServers.monk, leaving Antigravity disconnected even when the companion was healthy.

Validation

• & 'C:\Program Files\Git\usr\bin\bash.exe' -lc 'PATH=/usr/bin:/bin:$PATH; tests/start-monk-agent-antigravity-registration.sh'
• & 'C:\Program Files\Git\usr\bin\bash.exe' -lc 'PATH=/usr/bin:/bin:$PATH; sh -n tests/start-monk-agent-antigravity-registration.sh; sh -n scripts/start-monk-agent.sh; sh -n plugins/monk/scripts/start-monk-agent.sh; sh -n .antigravity-plugin/scripts/start-monk-agent.sh'
• git diff --check
• Root/plugin/Antigravity launcher copies are byte-identical.

Fixes #19

Prepared with Codex assistance and validated locally with Git Bash on Windows.