fix: execute ProxyCommand transport

[inureyes]

Summary

• Execute resolved ProxyCommand values as managed shell child transports instead of silently opening a direct target socket.
• Match OpenSSH proxy selection semantics across ProxyCommand, ProxyJump, command-line -J, explicit none, and effective %h/%k/%n token identities.
• Report bounded proxy stderr and exit status as actionable connection errors, and reject unsupported ProxyUseFdpass before spawning a process.

Implementation

• Add a duplex async transport backed by child stdin/stdout with bounded stderr draining, exit monitoring, deterministic shutdown, and kill-on-drop lifecycle handling.
• Expand %%, %h, %k, %n, %p, and %r, reject malformed or unsupported tokens, and preserve shell pipes and quoting from trusted SSH configuration.
• Preserve the first-obtained SSH configuration semantics introduced by the related proxy and host-key changes, with command-line -J taking precedence.
• Silence only the official but unimplemented SecurityKeyProvider option while retaining diagnostics for genuinely unknown keywords.
• Preserve raw command exit codes after stdout and stderr and keep draining the SSH channel when the local output receiver closes, fixing integration failures exposed by the real proxy transport.
• Update user and manual documentation to describe the supported behavior and the explicit ProxyUseFdpass limitation.

Security and quality review

• Reviewed shell-boundary validation, token expansion, direct-connection avoidance, child cleanup, stderr memory bounds, failure mapping, host-key alias identity, and early receiver-drop behavior; no unresolved CRITICAL or HIGH findings remain.
• Proxy commands remain an explicitly trusted SSH configuration shell boundary; target-derived tokens retain existing host and username validation, while NUL and newline injection and unsupported expansion tokens are rejected.

Validation

• cargo fmt --all -- --check
• cargo test --lib proxy (29 passed)
• cargo test --lib resolve_effective_jump_hosts (11 passed)
• cargo test --lib stream_exit (2 passed)
• cargo test --lib nonzero_stream_completion_preserves_remote_status (1 passed)
• cargo test --lib closed_output_receiver_is_not_a_command_error (1 passed)
• cargo test --test ssh_compat_output_test deprecated_alias_is_silent_and_unknown_keyword_uses_log_file -- --exact (1 passed)
• cargo check --lib --tests
• cargo clippy --lib --tests -- -D warnings
• cargo build --bin bssh
• cargo test --bin bssh (57 passed)
• TEST_SSH_UNSAFE_PERMISSIONS=1 python3 tests/openssh-regress/run.py --bssh target/debug/bssh --test proxy-connect --jobs 1 --timeout 120 (PASS, 1/1)

Local CI-equivalent note: cargo test --tests -- --skip integration_test passed the library tests (1420 passed, 9 ignored, 13 filtered), the bssh binary tests (57 passed), and the subsequent keygen/server and independent test targets until tests/integration_test.rs; three unchanged localhost SSH-dependent cases failed in the local harness (test_localhost_multiple_file_upload, test_parallel_execution_with_multiple_nodes, and test_download_with_unique_filenames). The PR's exact-head GitHub checks are the authoritative CI verdict for those environment-dependent cases.

Closes #280