feat(toolintent): reference-must-exist gate (anti-hallucination)

A file_read of a path that is inside the workspace but does not exist is now
BLOCKED with "no such path" instead of silently returning empty — so an agent
that hallucinated a filename fails loud and self-corrects (list the directory,
fix the name) rather than proceeding on an absent reference. Out-of-workspace
targets stay PathContainmentRule's concern, so the gates don't double-handle.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/server/src/main/kotlin/com/correx/apps/server/Main.kt

@@ -62,6 +62,7 @@ import com.correx.core.toolintent.rules.ManifestContainmentRule
 import com.correx.core.toolintent.rules.NetworkHostRule
 import com.correx.core.toolintent.rules.PathContainmentRule
 import com.correx.core.toolintent.rules.ReadBeforeWriteRule
+import com.correx.core.toolintent.rules.ReferenceExistsRule
 import com.correx.apps.server.freestyle.FreestyleDriver
 import com.correx.apps.server.inference.summarizeWithInference
 import com.correx.core.kernel.orchestration.JournalCompactionService
@@ -263,6 +264,7 @@ fun main() {
         rules = listOf(
             PathContainmentRule(),
             ReadBeforeWriteRule(),
+            ReferenceExistsRule(),
             ManifestContainmentRule(),
             ExecInterpreterRule(toolsConfig.interpreterExecutables.toSet()),
             NetworkHostRule(