release v0.6.1

[iicky]

• fix Node bindings broken since v0.6.0 — Vault::get and Vault::export now copy plaintext across the napi boundary instead of trying to return Zeroizing as String
• zeroize plaintext in cmd_edit, cmd_import, parse_env, and import_secrets so the in-memory window for unencrypted values is shorter
• align README, ROADMAP, THREAT_MODEL, SECURITY, and docs/quick-start with shipped behavior (TOFU pinning, default-closed ssh-rsa, Zeroizing coverage, command list)
• end the node/Cargo.toml version drift — workspace, node crate, and npm package are all 0.6.1
• bump dependencies: blake3 1.8.4→1.8.5, napi-derive 3.5.3→3.5.4, clap 4.6.0→4.6.1, libc 0.2.184→0.2.186, assert_cmd 2.2.0→2.2.1, napi 3.8.4→3.8.5, dependabot/fetch-metadata 3.0.0→3.1.0, actions/setup-node 6.3.0→6.4.0