Improve threat modeling mediated flow gates

[danyili2632]

Summary

• add mediated, sidecar, local IPC, and in-process flow modeling guidance to trust boundary discovery
• add trust model, communication type, and delegation context fields to DFD annotations
• add CI/CD artifact integrity gates for provenance, signature verification, tag immutability, digest pinning, and OIDC trust scope
• extend output with a flow annotation table for non-point-to-point architectures

Bounty

Addresses #1532.

I have read and agree to the CONTRIBUTING.md bounty terms. Preferred payment method can be provided privately after maintainer acceptance.

Validation

• git diff --check