chore(74-02): remove dead code and polish README for public release

- git rm: server.py (non-functional MCP stub), .test-status, .claude/.test-status, tmp/ovh_ssh_host_ed25519_key.pub
- .gitignore: add .test-status and src/ patterns
- README: fix typo (priviledge→privilege), fix broken link (secret-proxy.nix→agent-sandbox.nix), add Testing section

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>