fix(review): result_schema gadfly findings + go mod tidy

- Extracted applyResultSchema (panic-isolated like safeFinalNudge — a
  validator bug must never convert a successful run into a panic error)
- Blank corrective-round reply no longer clobbers the prior candidate
- result_schema_valid now fires on first-try conforming answers too
- Multi-phase runs log WARN + result_schema_skipped instead of a
  silent drop
- errors.As instead of the hand-rolled assertion; redundant guard
  removed; go.mod/go.sum tidied (the CI failure)

Co-Authored-By: Claude Fable 5 <[email protected]>

go.mod

@@ -6,6 +6,7 @@ require (
 	gitea.stevedudenhoeffer.com/steve/majordomo v0.0.0-20260626223738-1fd7109a42f3
 	github.com/google/uuid v1.6.0
 	github.com/robfig/cron/v3 v3.0.1
+	github.com/santhosh-tekuri/jsonschema/v6 v6.0.2
 	golang.org/x/crypto v0.53.0
 	gopkg.in/yaml.v3 v3.0.1
 )
@@ -19,7 +20,6 @@ require (
 	github.com/google/s2a-go v0.1.8 // indirect
 	github.com/googleapis/enterprise-certificate-proxy v0.3.4 // indirect
 	github.com/gorilla/websocket v1.5.3 // indirect
-	github.com/santhosh-tekuri/jsonschema/v6 v6.0.2 // indirect
 	go.opencensus.io v0.24.0 // indirect
 	golang.org/x/net v0.55.0 // indirect
 	golang.org/x/sys v0.46.0 // indirect