docs: the contract manifest stands alone; public docs describe this engine only

The manifest's prose pointer named a document that answers a confidential
specification, and such a document does not belong in a public repository even
in summary. The pointer is dropped — the manifest is generated from this
engine's own surface and is self-describing — and the requirement marking it
deliberately omits is recorded with the contract's owner rather than here.

The standard is written down so this is not relitigated per document.

CONTRIBUTING.md

@@ -64,6 +64,10 @@ see `package.json` for `test:integration`, `test:coverage`, and friends.
   store, measured solo under an exclusive lock" tells a reader everything the
   number depends on; the machine it ran on and whose data it was tell them
   nothing except where somebody's infrastructure lives.
+- **Documents that answer or reference a confidential specification never enter
+  this repository, even summarized.** The public docs describe THIS engine and
+  the published contract, and nothing else — a summary of a private document is
+  still that document's contents.
 
 ## License