fix: render const values as literals in output format instruction (#129)

buildOutputFormatInstruction now renders const fields with their actual
value (e.g. $status: greeted) instead of the type placeholder (<string>).
Also adds early return in resolvePropertySchema for const properties.

Fixes #129

.changeset/const-prompt-literal.md

@@ -0,0 +1,9 @@
+---
+"@united-workforce/util-agent": patch
+---
+
+fix: render const values as literals in output format instruction (#129)
+
+Previously `buildOutputFormatInstruction` rendered `const: greeted` as
+`$status: <string>`, causing agents to output `$status: const` instead of
+the actual value. Now const fields render as `$status: greeted  # required | fixed value`.