fix(porch-store): add the missing use json import

- docs/examples/porch/ did not typecheck: WO-E403 "cannot resolve the
  receiver's type for the call to `encode`" on json.encode
- every other example that calls json.encode/decode imports it; this
  file did not, so the whole porch library was uncompilable on dev
- woc docs/examples/porch/ now exits 0

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
(cherry picked from commit 5c3544d524fa98dbb7a363600cd2eeb6dd1badac)

docs/examples/porch/middleware/idempotent.wo

@@ -5,6 +5,7 @@
 
 use time
 use http
+use json
 
 -- Idempotent middleware: before (check/replay) + after (store on miss).
 -- Key composition: "idem:${header}" or "idem:${header}:${sha256(method|path|body)}"