docs(rules): add an unable-to-verify reporting standard

verification.md required running tests/lint/typecheck/build before claiming done, but said nothing about what to do when a command can't run. A new "When You Cannot Verify" section requires a four-part report (command attempted, why it couldn't run, risk left unverified, and the smallest next command for the user) and states the principle that a check that didn't run is never reported as a pass.

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