feat(bsshd): support publickey exec sessions

[ryanbreen]

Summary

• records SSH exec requests in the server channel state
• runs non-interactive bsshd exec commands with stdout/stderr relayed over the SSH channel
• sends SSH exit-status before closing the channel

Validation

• cargo check --manifest-path libs/libbreenix/Cargo.toml --features std
• userspace/programs/build.sh --arch aarch64
• cargo build --release --features testing,external_test_bins --bin qemu-uefi (warning scan empty)
• macOS OpenSSH publickey-only proof with StrictHostKeyChecking=yes and UserKnownHostsFile=turn92-artifacts/known_hosts:
  • Authenticated using /Users/wrb/.ssh/id_rsa publickey
  • exec request accepted
  • stdout included BREEMARK and Breenix uname output
  • SSH exit status 0

Artifacts: /Users/wrb/Downloads/Ralph/breenix-interrupt-io-roadmap-1780056222/turn92-artifacts