feat(l1): proposal L1-source block + engineer L1-escalations section

- flow-proposal.ts: source_session_id nullable + add l1_session_id (matches backend
  FlowProposalSummary).
- ProposalDetail.tsx: render an 'AI L1 walk (outcome-validated)' note when
  l1_session_id is set instead of the /pilot/{source_session_id} link; fall back to
  the link for ai_session-sourced proposals.
- New L1EscalationsSection.tsx (GET /l1/escalations) — expandable rows with walked-path
  summary; renders nothing if empty. Mounted below the FlowPilot queue on
  EscalationQueuePage. tsc -b + eslint clean.

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

frontend/src/components/l1/L1EscalationsSection.tsx

@@ -0,0 +1,77 @@
+import { useEffect, useState } from 'react'
+import { l1Api } from '@/api/l1'
+import type { WalkSession } from '@/types/l1'
+
+/**
+ * Engineer-visible list of escalated L1 sessions (the Phase 2A handoff queue).
+ * Backed by GET /l1/escalations (engineer-or-above). Pollable, dependency-free —
+ * each row expands to show the walked path summary. Renders nothing if empty.
+ */
+export function L1EscalationsSection() {
+  const [rows, setRows] = useState<WalkSession[]>([])
+  const [loaded, setLoaded] = useState(false)
+  const [expanded, setExpanded] = useState<string | null>(null)
+
+  useEffect(() => {
+    l1Api
+      .escalations()
+      .then(setRows)
+      .catch(() => setRows([]))
+      .finally(() => setLoaded(true))
+  }, [])
+
+  if (!loaded || rows.length === 0) return null
+
+  return (
+    <section className="space-y-3">
+      <div>
+        <h2 className="font-heading text-lg font-bold text-heading">L1 escalations</h2>
+        <p className="text-sm text-text-muted">
+          Tickets an L1 tech escalated mid-walk — pick one up to continue.
+        </p>
+      </div>
+      <div className="rounded-lg border border-default bg-card overflow-hidden">
+        {rows.map((s) => {
+          const isOpen = expanded === s.id
+          return (
+            <div key={s.id} className="border-b border-default last:border-b-0">
+              <button
+                type="button"
+                onClick={() => setExpanded(isOpen ? null : s.id)}
+                className="w-full px-4 py-3 flex items-center justify-between text-left hover:bg-elevated transition-colors"
+              >
+                <div className="flex items-center gap-3 min-w-0">
+                  <span className="font-mono text-xs text-text-muted">#{s.id.slice(0, 8)}</span>
+                  <span className="text-sm text-text-primary truncate">
+                    {s.walked_path.length} step{s.walked_path.length === 1 ? '' : 's'} walked
+                  </span>
+                </div>
+                <span className="text-xs text-text-muted whitespace-nowrap">
+                  {new Date(s.last_step_at).toLocaleString()}
+                </span>
+              </button>
+              {isOpen && (
+                <div className="px-4 pb-3 space-y-1.5">
+                  {s.walked_path.length === 0 ? (
+                    <p className="text-xs text-text-muted">No steps recorded.</p>
+                  ) : (
+                    <ol className="space-y-1.5 text-sm">
+                      {s.walked_path.map((step, i) => (
+                        <li key={i} className="flex flex-col">
+                          <span className="text-text-muted text-xs">{step.question}</span>
+                          {step.answer && (
+                            <span className="font-medium text-text-primary">→ {step.answer}</span>
+                          )}
+                        </li>
+                      ))}
+                    </ol>
+                  )}
+                </div>
+              )}
+            </div>
+          )
+        })}
+      </div>
+    </section>
+  )
+}

frontend/src/types/flow-proposal.ts

@@ -10,7 +10,10 @@ export interface FlowProposalSummary {
   supporting_session_count: number
   status: 'pending' | 'approved' | 'modified' | 'rejected' | 'dismissed' | 'auto_reinforced'
   target_flow_id: string | null
-  source_session_id: string
+  // Exactly one source is set: source_session_id (FlowPilot ai_session) XOR
+  // l1_session_id (L1 ai_build walk). Both nullable on the backend (Phase 2A).
+  source_session_id: string | null
+  l1_session_id: string | null
   created_at: string
 }