Fix nested tmux selection handling

[acmore]

Summary

• detect when okdev ssh is launched from a local tmux session and disable mouse mode in the inner okdev tmux session
• document the shared mouse-mode behavior for clients attached to the same remote tmux session
• document that okdev exec -- <cmd> is non-interactive fanout and that TTY-dependent tools like watch should use okdev ssh or an interactive okdev exec shell

Test Plan

• go test ./internal/cli ./cmd/okdev-sshd ./internal/ssh
• .venv/bin/pre-commit run --all-files --hook-stage manual okdev-gofmt