fix: update default model from claude-sonnet-4-5 to claude-sonnet-4-6 (#1390)

CLAUDE_MEM_MODEL defaulted to the deprecated claude-sonnet-4-5 across source,
installer, tests, and documentation. Updated all references to claude-sonnet-4-6.

Closes #1390

Co-Authored-By: Claude <noreply@anthropic.com>

docs/public/architecture/hooks.mdx

@@ -860,7 +860,7 @@ async startSession(session: ActiveSession, worker?: any) {
   const queryResult = query({
     prompt: messageGenerator,
     options: {
-      model: 'claude-sonnet-4-5',
+      model: 'claude-sonnet-4-6',
       disallowedTools: ['Bash', 'Read', 'Write', ...],  // Observer-only
       abortController: session.abortController
     }