Rewrite around v2 agent ownership model

[posix4e]

Summary

• Delete the v1 control-plane/fleet implementation, old EasyEnclave app bundle, website files, and giant CI/deploy workflows.
• Add a minimal dd-agent crate around the v2 model: runtime assignment, current-owner-only deploy/log/exec, GitHub OIDC principal checks, and public /health proof.
• Add lightweight assign/deploy/verify composite actions and replacement Rust CI.
• Add v2 product spec, threat model, and rewrite plan docs.

Validation

• cargo fmt --all -- --check
• cargo check --workspace
• cargo test --workspace

Notes

This is intentionally a destructive draft PR. It removes v1 instead of keeping a compatibility layer so the new ownership model can drive the project shape.