Two private `isObject` copies duplicate the exported `isJsonObject` guard

[Deepam02]

Problem

claude/json-file.ts:33 already exports a plain-object type guard:

export function isJsonObject(value: unknown): value is JsonObject {
  return typeof value === "object" && value !== null && !Array.isArray(value);
}

The identical predicate is reimplemented as a private isObject in two other modules:

• packages/agentctx/src/mcp/server.ts:152
• packages/agentctx/src/profile/detect.ts:295

Both have the exact same body. (extract/schema.ts:172 has a closely related asObject that returns the object-or-null — worth a glance but it has a different return shape, so it may stay separate.) Same "duplicated helper" cleanup family as #89 / #90.

What done looks like

• Have mcp/server.ts and profile/detect.ts import and use the existing isJsonObject instead of their local isObject (the JsonObject vs Record<string, unknown> narrowing is compatible for these call sites — adjust types if needed).
• Delete the two private copies.
• npm run check stays green.

Why it's a good first issue

Small, mechanical de-duplication confined to two files, with an already-exported helper to reuse.