Document the commercial entitlement boundary

[turinglambdaai]

Summary

• document product-level composition with Permit in both READMEs
• make the no-framework-dependency rule persistent for future maintainers and agents
• record secure-storage and signing-key trust boundaries in the architecture owner

Rivet does not depend on Permit and no licensing implementation is added.

Validation

• isolated package install
• raco test rivet/tests/documentation.rkt (626 tests)
• git diff --check

[turinglambdaai]

Integration note: this PR changes documentation and repository guidance only. Its focused documentation suite passed locally (626 checks). The Kotlin protocol/runtime failure is inherited from current main, where the same job fails on commit 0b7fc5c (main CI run 38775134709); it is unrelated to the Permit boundary. No Rivet runtime, protocol, UI, packaging, or dependency code is changed here.