fix: preserve non-object Antigravity configs

[aikinley]

Summary

• validate Antigravity's existing MCP config as a JSON object, not only as syntactically valid JSON
• preserve valid non-object configs and keep the healthy launcher running with a warning
• apply the fix to all three shipped launcher mirrors
• add a red-to-green regression for the Python fallback path

Why

A valid JSON array passes the current syntax check, then the Python fallback calls cfg.get(...) and exits under set -e. The same incompatible shape also breaks the jq assignment path. A healthy monk-agent therefore appears to fail solely because an existing Antigravity config has the wrong root type.

Validation

• C:\Program Files\Git\bin\bash.exe tests/start-monk-agent-antigravity-nonobject-config.sh
• C:\Program Files\Git\bin\bash.exe -n scripts/start-monk-agent.sh
• SHA-256 equality verified across all three launcher mirrors
• git diff --check

The regression fails on pristine v0.1.54 with AttributeError: 'list' object has no attribute 'get' and passes with this change while preserving the original config.

Generated artifacts

GENERATED.md says the public tree is generated from the private monk-agent/plugin source. This PR updates the mirrored public artifacts consistently; the corresponding source change should be ported before the next regeneration.

Fixes #255