feat: add test_stub optional field to specialist finding schema

All specialist prompts now document test_stub as an optional output field,
enabling specialists to suggest test code alongside findings.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

review/specialists/api-contract.md

@@ -3,6 +3,7 @@
 Scope: When SCOPE_API=true
 Output: JSON objects, one finding per line. Schema:
 {"severity":"CRITICAL|INFORMATIONAL","confidence":N,"path":"file","line":N,"category":"api-contract","summary":"...","fix":"...","fingerprint":"path:line:api-contract","specialist":"api-contract"}
+Optional: line, fix, fingerprint, evidence, test_stub.
 If no findings: output `NO FINDINGS` and nothing else.
 
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