Clarify schema-backed fact retraction contract

[justinjoy]

Summary

• Document that HTTP fact append/retract/forget operate on schema-registered fact-store relations, not raw Datalog atoms.
• Add a unary fact(V) example showing how to model fact(1), fact(2), fact(3) as a registered fact(value:int64) relation and retract fact(1).
• Add HTTP integration coverage for missing-schema retract and schema-backed unary append/query/retract/query behavior.

Closes #328

Review flow

• Architect and critic agreed not to implement raw atom deletion or /api/facts/fact(1), because that would require a separate parser, typing, authorization, idempotency, audit, and replay contract.
• Implementer produced one atomic docs/test commit.
• Reviewer reported no findings.
• Architect and critic both approved the final commit.

Test

• git diff --check
• meson test -C builddir-facts daemon-http-facts --print-errorlogs