feat: add target repo examples and file layout guidance

examples/rust-project/ai/harness-health.md

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+# Harness Health
+
+- project map clarity: partial
+- validation commands: strong
+- fast feedback: partial
+- task templates: partial
+- guardrails: strong
+- doc-code alignment: partial
+
+## Top priorities
+1. Document workspace crate roles more clearly
+2. Call out feature-flag-specific validation paths
+3. Keep examples aligned with actual crate boundaries