fix(content): version-gate the Next.js client init rule

instrumentation-client.ts is ignored on Next.js below 15.3, so client-side
PostHog silently never initializes there. The nextjs commandment now states
which init location to use for which version.

Generated-By: PostHog Code
Task-Id: f3b2162f-9a47-4d22-a83e-0a66900ed730

Author: gewenyu99

context/commandments.yaml

@@ -24,7 +24,7 @@ commandments:
 
   nextjs:
     # Initialization
-    - For Next.js 15.3+, initialize PostHog in instrumentation-client.ts for the simplest setup
+    - 'Initialize client-side PostHog in exactly one place, based on the "next" version in package.json: on 15.3 and later, use instrumentation-client.ts; below 15.3, use a "use client" PostHogProvider in the root layout (or _app), because instrumentation-client.ts is ignored there'
 
   nextjs-feature-flags:
     # Client-side feature flags