Support interactive SSH sessions on Windows

[htr]

Summary

• Fall back to pipe-backed stdin, stdout, and stderr when an SSH client requests a PTY on Windows.
• Connect stdin for non-PTY SSH sessions.
• Add a Windows integration test that starts the SSH server, requests a PTY, and runs commands through cmd.exe.
• Run the SSH package test on windows-latest in commit checks.

Validation

• go test ./...
• GOOS=windows GOARCH=amd64 go test -c -o /tmp/sshd-windows.test.exe ./pkg/sshd
• GOOS=windows GOARCH=amd64 go build -o /tmp/breakpoint-windows.exe ./cmd/breakpoint
• Windows SSH and code checks: https://github.com/namespacelabs/breakpoint/actions/runs/30365446353