fix(cli)!: restore OpenSSH short flag semantics

[inureyes]

Summary

• restore OpenSSH meanings for -N, -f, -C, -A, -S, -k, and -b in single-destination mode while retaining bssh extensions as long options and pdsh meanings in pdsh mode
• complete the connection-multiplexing lifecycle with authenticated background workers, ControlPersist, forwarding control commands, and no-session/background execution
• implement session-scoped SSH agent forwarding, authentication banners, quiet diagnostics, query aliases, and configuration/runtime wiring needed by the compatibility suite
• accept repeated -E options and -O proxy, preserve the OpenSSH -F /dev/null convention without weakening Include validation, and keep transfer fixtures independent of platform debug-symbol size
• update migration guidance, examples, man pages, measured compatibility documentation, and the committed Linux baseline

Compatibility result

The pinned OpenSSH V_10_3_P1 harness reports 60/66 passes on macOS and 61/69 on Ubuntu in CI. Both platforms reach the epic's 60-pass gate. Remaining failures are documented in the uploaded result tables and are not concealed by lowering the eligible-result floor.

The requested published warning cycle in an earlier 2.x release cannot be created retroactively. This change does emit bounded migration warnings on interactive standard error, documents every long-option rewrite, and does not claim that a prior release contained those warnings. Redirected protocol streams and -E logs remain byte-transparent.

Validation

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --lib (1,699 passed, 7 ignored)
• cargo test --test log_file_test (5 passed)
• cargo test --test agent_forwarding_live_test -- --nocapture (6 passed)
• cargo test --test control_multiplexing_live_test -- --nocapture (9 passed)
• make openssh-regress-selftest (14 passed)
• python3 tests/openssh-regress/run.py --test agent (1 passed)
• python3 tests/openssh-regress/run.py --test sftp (1 passed with the stable 1 MiB fixture)
• python3 tests/openssh-regress/run.py --test forward-control --test sshsig (2 passed with audited 180-second minimums)
• GitHub CI: macOS OpenSSH harness (60 passed, 6 failed, 7 environmental, 6 upstream skips)
• GitHub CI: Ubuntu OpenSSH harness (61 passed, 8 failed, 4 environmental, 6 upstream skips)

The full all-target Rust run has three environment-dependent localhost SSH authentication failures. The same three failures reproduce on the base main commit; all other targets pass.

Closes #286

Closes #287

Part of #275