Feature request: Proof of non-disclosure

Keybase should provide a mechanism, such that:
1) A user can attest non-disclosure of their private PGP key to keybase
2) Keybase affirms the claim by validating the lack of private key data in their database
3) Keybase automatically negates the affirmation and attestation upon receipt of private-key data.

Reason: In ring0 security circles, simply utilizing keybase is equivalent to disclosure of the private key, due to there being no mechanism to ensure that the key was not leaked by a third-party.

Issue #1619 names no files, tests, or entry points. Start by defining how the attestation is represented, how Keybase validates the absence of private-key data, and how receiving such data automatically invalidates it. Done requires an agreed design and tests covering attestation, affirmation, and negation.

Written by the indexing model from the issue text.