fix: re-apply selfImprovement disabled in sessionDefault/Enabled manifest test configs

Lost during rebase onto master. selfImprovement now defaults to enabled,
so these test configs need it explicitly disabled to isolate command:new assertions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Coke1120

test/plugin-manifest-regression.mjs

@@ -171,6 +171,7 @@ try {
     dbPath: path.join(workDir, "db-session-default"),
     autoCapture: false,
     autoRecall: false,
+    selfImprovement: { enabled: false },
     sessionMemory: {},
     embedding: {
       provider: "openai-compatible",
@@ -191,6 +192,7 @@ try {
     dbPath: path.join(workDir, "db-session-enabled"),
     autoCapture: false,
     autoRecall: false,
+    selfImprovement: { enabled: false },
     sessionMemory: { enabled: true },
     embedding: {
       provider: "openai-compatible",