feat: plan_snapshot field on ClaudeJob + architecture doc (#23)

* feat: add plan_snapshot field to ClaudeJob schema

Nullable String? column on claude_jobs captures the task's
implementation_plan at claim time — immutable baseline for drift detection.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs: update ClaudeJob lifecycle with plan_snapshot

Document state machine snapshot capture/reset, plan_snapshot field
rationale, and drift-detection baseline semantics.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* refactor: remove duplicate header labels on backlog page

Both the product H1 + description in the page header and the
"Product Backlog" panel-title in the PBI panel duplicated info
already visible in the NavBar. Removed both, keeping the right-aligned
action bars (activate/sprint/settings, plus filters/+PBI) intact.

PanelNavBar component is unchanged — Stories and Taken panels keep
their titles.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

app/(app)/products/[id]/page.tsx

@@ -92,14 +92,8 @@ export default async function ProductBacklogPage({ params, searchParams }: Props
 
   return (
     <div className="flex flex-col h-full">
-      {/* Product header */}
-      <div className="px-4 py-3 border-b border-border bg-surface-container-low shrink-0 flex items-center justify-between">
-        <div>
-          <h1 className="font-medium text-foreground">{product.name}</h1>
-          {product.description && (
-            <p className="text-xs text-muted-foreground mt-0.5">{product.description}</p>
-          )}
-        </div>
+      {/* Product header — actions only; product-naam zit al in NavBar */}
+      <div className="px-4 py-3 border-b border-border bg-surface-container-low shrink-0 flex items-center justify-end">
         <div className="flex items-center gap-3">
           {user?.active_product_id !== id && (
             <ActivateProductButton productId={id} isDemo={isDemo} redirectTo={`/products/${id}`} />