fix(subagent): harden continuable persistence

examples/acp-agent/tests/fixtures/subagent-durability-failure.ts

@@ -0,0 +1,14 @@
+import type { Context } from 'cordis'
+
+export const name = 'subagent-durability-failure'
+
+/** Fail a continuable child's provider-owned final durability confirmation. */
+export function apply(ctx: Context): void {
+  const flushedTurnEnds = new WeakSet<object>()
+  ctx.on('session/flush', (session) => {
+    if (session.header.parentSession === undefined) return
+    if (session.events.at(-1)?.type !== 'turn/end') return
+    if (flushedTurnEnds.has(session)) throw new Error('snapshot disk full')
+    flushedTurnEnds.add(session)
+  })
+}