fix(ssh): enable macOS UseKeychain runtime support

[inureyes]

Summary

• Treat Apple's macOS UseKeychain directive as runtime-supported authentication instead of emitting a false unsupported-option warning, and expose the resolved value through bssh -G.
• Fall back to terminal input when a stored Keychain passphrase is stale, and store newly entered passphrases only after successful private-key decryption.
• Document bssh's separate Keychain namespace and add parser, resolver, configuration-dump, and encrypted-key regression coverage.

Testing

• cargo fmt --all --check
• cargo clippy -- -D warnings
• cargo test --lib use_keychain (14 passed)
• cargo test --test ssh_config_dump_test (22 passed)
• target/debug/bssh -G usekeychain-probe.invalid with the user's default SSH configuration (exit 0, empty stderr, usekeychain yes)

Compatibility notes

Apple OpenSSH stores passphrases in the entitlement-protected com.apple.ssh.passphrases access group. bssh cannot reuse those records directly, so it stores passphrases under its own bssh-ssh-key-passphrase service and may prompt once on the first bssh connection.

A broader cargo test --lib run completed with 1,714 passing tests and four failures in unrelated jump-authentication, control-socket permission, and known-hosts timeout tests. The Linux cross-check could not reach bssh code because the host lacks x86_64-linux-gnu-gcc required by aws-lc-sys.