feat: register $first/$last JSONata functions in moderator

Register custom $first(role) and $last(role) functions in the JSONata
evaluator. These search the steps array and return the matching role's
frontmatter (output) directly, replacing verbose steps[-1].output.x
expressions with semantic $last('role').field syntax.

- workflow-moderator: register functions via expr.registerFunction()
- Updated all condition expressions in .workflows/ and examples/
- Added tests for $last, $first, and unmatched role (undefined)

Fixes #376

examples/solve-issue.yaml

@@ -57,7 +57,7 @@ roles:
 conditions:
   notApproved:
     description: "Reviewer rejected the implementation"
-    expression: "steps[-1].output.approved = false"
+    expression: "$last('reviewer').approved = false"
 graph:
   $START:
     - role: "planner"