Dump the router prompt where the labeler can read it (V-661)

The training workspace labels with routeSystem and routeGrammar, and it held
its own copies. V-660 changed both. A retyped prompt drifts silently, which is
the problem llm/check_prompt_parity.py exists for on the other side.

Inert unless MAVEN_DUMP_PROMPT names a directory.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013ptwopxyo3Z2kwFckHkLvN

internal/router/dumpprompt_test.go

@@ -0,0 +1,22 @@
+package router
+
+import (
+	"os"
+	"testing"
+)
+
+// TestDumpPrompt writes the router prompt and grammar to disk so the training
+// workspace labels with the daemon's own contract rather than a retyped copy.
+// It is inert unless MAVEN_DUMP_PROMPT names a directory.
+func TestDumpPrompt(t *testing.T) {
+	dir := os.Getenv("MAVEN_DUMP_PROMPT")
+	if dir == "" {
+		t.Skip("MAVEN_DUMP_PROMPT unset")
+	}
+	if err := os.WriteFile(dir+"/route_system.txt", []byte(routeSystem), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(dir+"/route_grammar.gbnf", []byte(routeGrammar), 0o644); err != nil {
+		t.Fatal(err)
+	}
+}