feat(l1): L1WalkPage tree variant with Resolve/Escalate modals

Replaces the T20 stub. WalkPage dispatches by session_kind:
- 'flow' / 'proposal' → L1WalkTreeVariant (this commit)
- 'adhoc' → placeholder until T23

L1WalkTreeVariant: sticky header with back link + AI-built badge +
persistent Escalate/Resolve buttons; two-pane body (current step
yes/no card on left, walked-path transcript on right). ResolveModal
and EscalateModal extracted to shared WalkModals.tsx (T23 reuses).

Phase 1 caveat: this surface isn't reached by user-driven intake
(which creates adhoc sessions only). It's exercised via direct URL
or integration tests until Phase 2 wires match_or_build.

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

frontend/src/pages/l1/L1WalkPage.tsx

@@ -1,13 +1,68 @@
+import { useEffect, useState } from 'react'
+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 type { WalkSession } from '@/types/l1'
 
 export default function L1WalkPage() {
-  return (
-    <div className="overflow-y-auto h-full">
-      <PageMeta title="L1 Walk" />
-      <div className="max-w-4xl mx-auto px-6 pt-12 pb-12">
-        <h1 className="font-heading text-2xl font-bold">L1 Walk</h1>
-        <p className="text-muted-foreground mt-2">Loading…</p>
+  const { sessionId } = useParams<{ sessionId: string }>()
+  const navigate = useNavigate()
+  const [session, setSession] = useState<WalkSession | null>(null)
+  const [error, setError] = useState<string | null>(null)
+
+  useEffect(() => {
+    if (!sessionId) return
+    l1Api.getSession(sessionId)
+      .then(setSession)
+      .catch((err) => {
+        const msg = err?.response?.data?.detail || err?.message || 'Failed to load session'
+        setError(typeof msg === 'string' ? msg : 'Failed to load session')
+      })
+  }, [sessionId])
+
+  if (error) {
+    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">
+          {error}
+        </div>
       </div>
-    </div>
+    )
+  }
+  if (!session) {
+    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">Loading…</div>
+      </div>
+    )
+  }
+
+  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.
+  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>
+    )
+  }
+
+  return (
+    <>
+      <PageMeta title="L1 Walk" />
+      <L1WalkTreeVariant
+        session={session}
+        onSessionUpdate={setSession}
+        onDone={handleDone}
+      />
+    </>
   )
 }