feat(keygen): enroll FIDO security keys and load resident keys #337

Problem / Background

bssh signs with FIDO sk-* keys (#333, #334) but cannot create them, so users still need OpenSSH ssh-keygen to produce id_ed25519_sk / id_ecdsa_sk. bssh-keygen accepts only ed25519 and rsa (src/bin/bssh_keygen.rs:98, src/keygen/mod.rs:69,94). SecurityKeyProvider (src/ssh/security_key/mod.rs:179) has only sign. The external path checks that sk_enroll and sk_load_resident_keys resolve but never calls them (src/ssh/security_key/external.rs:18,186). The internal provider encodes makeCredential and U2F REGISTER only as dummy touch probes for device selection (internal/ctap2.rs:395, internal/u2f.rs:76).

Proposed Solution

Add enroll and load_resident_keys to SecurityKeyProvider, implement them in both providers, and wire them into bssh-keygen with OpenSSH's flags: -t ed25519-sk|ecdsa-sk, -w provider (resolved through resolve_provider, default internal), -K, and -O application=, resident, verify-required, no-touch-required, user=, challenge=, write-attestation=, device=. Write files with ssh-key (KeypairData::SkEd25519 / SkEcdsaSha2NistP256), the same types security_key_signer.rs:76 reads.

Out of scope: an ssh-add -K equivalent. bssh ships no agent client tool and reads key files directly; -K output files cover the use case.

Acceptance Criteria

• bssh-keygen -t ed25519-sk and -t ecdsa-sk enroll through the external library and internal, writing private (key handle) and public files that OpenSSH ssh and bssh both accept.
• Each listed -O option works or is rejected with a message naming it; ed25519-sk on a U2F-only device fails clearly.
• bssh-keygen -K writes one key pair per resident credential using OpenSSH's id_*_sk_rk* naming, prompting for the PIN.
• Internal provider implements CTAP2 makeCredential, credential management with PIN, and U2F registration for ecdsa-sk, with tests in the mock_handlers.rs style.
• External path calls sk_enroll / sk_load_resident_keys with the pointer and length checks sk_sign uses, tested in external_tests.rs.
• CLI help and docs/man/bssh-keygen.1 document the new types and options.
• Manual check with a physical authenticator, pending user validation if none is available.

Verification

cargo test --lib security_key
cargo clippy --all-targets -- -D warnings
cargo fmt --check

Implementation Notes

Built-in resident enrollment requires CTAP2 credential management and a PIN to prevent replacing an existing application/user credential; resident and verification-required keys require credProtect. The write-attestation option is explicitly rejected, and ssh-key serialization limits credential handles to 255 bytes.

Physical-authenticator validation remains pending: this environment has no HID authenticator.