Add isolated v2 agent ownership prototype

[posix4e]

Summary

• Keep the existing v1 runtime, EasyEnclave integration, apps, website, and fleet CI intact.
• Add an isolated crates/dd-agent prototype for the v2 ownership model: runtime assignment, current-owner-only deploy/log/exec, GitHub OIDC principal checks, and public /health proof.
• Add prototype assign/deploy/verify composite actions for external workload repos.
• Add a small CI workflow that validates the full workspace, including both v1 and the prototype.
• Keep only the threat-model doc; remove the spec/rewrite-plan docs from the PR.

Validation

• cargo fmt --all -- --check
• cargo check --workspace
• cargo test --workspace
• local smoke test: cargo run -p dd-agent and GET /health

Notes

This is now intentionally non-destructive. The v2 agent is not wired into release/deploy paths and is not feature-complete. Missing pieces before replacement: TDX quote minting, EasyEnclave execution backend, Cloudflare ingress, and managed assignment reconciliation.