Bump plugin to 2.39.0 with features from Matt Van Horn (@mvanhorn):
context budget precheck, plan sequence numbers, review serial mode,
agent-browser debugging commands, test-browser port detection, lfg
phase gating, and Context7 API key auth.
Also fixes MCP server merge order so plugin servers correctly
overwrite stale entries during sync.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>