Agent Skills: smith-preflight

Pre-ship gate — check the current change against the smith invariants (branch-first, secret-scan, pr-ownership, external-write, verify-before-assert, suggestions, subagent-contract) and report PASS, FAIL, SKIP or N/A (not applicable) per check with a GO or NO-GO verdict. Invoke with /smith-preflight; /smith-ship runs it as step 0 and will not push on NO-GO.