feat(plugin): release v2.39.0 with community contributions

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>

src/sync/json-config.ts

@@ -18,8 +18,8 @@ export async function mergeJsonConfigAtKey(options: {
   const merged = {
     ...existing,
     [key]: {
-      ...incoming,
-      ...existingEntries, // existing user entries win on conflict
+      ...existingEntries,
+      ...incoming, // incoming plugin entries overwrite same-named servers
     },
   }