feat(l1): adhoc walker variant with debounced notes autosave

The session variant that Phase 1 L1 users actually hit (intake creates
adhoc sessions when no flow_id is provided). Single-pane note-taking
surface with 300ms-debounced autosave to walk_notes. Shares header
shape + Resolve/Escalate modals with the tree variant. Splits the
notes textarea by paragraph and persists each as a structured
AdhocNote entry. Stops saving once status leaves 'active'.

L1WalkPage now dispatches both variants.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/pages/l1/L1WalkPage.tsx

@@ -3,6 +3,7 @@ import { useParams, useNavigate } from 'react-router-dom'
 import { PageMeta } from '@/components/common/PageMeta'
 import { l1Api } from '@/api/l1'
 import { L1WalkTreeVariant } from '@/components/l1/L1WalkTreeVariant'
+import { L1WalkAdhocVariant } from '@/components/l1/L1WalkAdhocVariant'
 import type { WalkSession } from '@/types/l1'
 
 export default function L1WalkPage() {
@@ -42,16 +43,17 @@ export default function L1WalkPage() {
 
   const handleDone = () => navigate('/l1')
 
-  // Phase 1: adhoc variant (T23) handles session_kind='adhoc'. Tree variant handles flow/proposal.
-  // For T22, only the tree variant is implemented. Adhoc sessions render a placeholder until T23 lands.
+  // Phase 1: adhoc variant handles session_kind='adhoc'. Tree variant handles flow/proposal.
   if (session.session_kind === 'adhoc') {
     return (
-      <div className="overflow-y-auto h-full">
+      <>
         <PageMeta title="L1 Walk" />
-        <div className="max-w-4xl mx-auto px-6 pt-12 text-muted-foreground">
-          Ad-hoc walker pending (T23).
-        </div>
-      </div>
+        <L1WalkAdhocVariant
+          session={session}
+          onSessionUpdate={setSession}
+          onDone={handleDone}
+        />
+      </>
     )
   }