fix(terminal): add SSH preflight check

[teknium1]

Summary

• add an SSH preflight check that fails fast with a clear error when the ssh client is unavailable
• keep the change narrowly scoped to tools/environments/ssh.py
• add focused regression tests covering the missing-SSH and happy-path cases

Notes

• salvages the substantive fix from PR fix(terminal): add SSH preflight check #1461 onto current main
• preserves contributor authorship by reimplementing the intended narrow fix without the stale unrelated setup.py changes from the original branch

Test plan

• python -m py_compile tools/environments/ssh.py tests/tools/test_ssh_environment.py
• python -m pytest tests/tools/test_ssh_environment.py tests/tools/test_terminal_requirements.py -n0 -q

Fixes #1458
Supersedes #1461