feat: Flag to print SSH cert and private key rather than FS

[EthanHeilman]

Adds a flag, -p or --print-key, to opkssh login so that the SSH cert and private key will be printed to output rather than writing these to the filesystem.

  -p, --print-key                 Print private key and SSH cert instead of writing them to the filesystem

.\opkssh.exe login --print-key
time="2026-01-04T16:24:45-05:00" level=info msg="Opening browser to http://localhost:53245/chooser"
time="2026-01-04T16:24:47-05:00" level=info msg="listening on http://127.0.0.1:3000/"
time="2026-01-04T16:24:47-05:00" level=info msg="press ctrl+c to stop"
ecdsa-sha2-nistp256-cert-v01@openssh.com AAAAKGVjZ...
-----BEGIN OPENSSH PRIVATE KEY-----
b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAaAAAABNl
...
ESfGyHEaJy9fl2oAAAAPb3BlbnB1YmtleSBjZXJ0AQ==
-----END OPENSSH PRIVATE KEY-----

Keys generated for identity
Email, sub, issuer, audience:
a...@gmail.com 12348 https://accounts.google.com 206584157355-7cbe4s640tvm7naoludob4ut1emii7sf.apps.googleusercontent.com

[Copilot]

Pull request overview

This PR adds a --print-key (-p) flag to the opkssh login command that allows users to print the SSH certificate and private key to standard output instead of writing them to the filesystem.

Key changes:

• Added a new boolean flag PrintKeyArg to control output destination for SSH credentials
• Modified the login flow to conditionally print keys to stdout or write to filesystem based on the flag
• Added test coverage for the new print-to-output functionality

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
main.go	Added printKeyArg flag variable, registered the -p/--print-key flag, and passed it to the NewLogin constructor
commands/login.go	Added PrintKeyArg field to LoginCmd struct, updated constructor signature, added out() helper method for output destination, and modified login flow to print keys when flag is set
commands/login_test.go	Added test case for print-key functionality with test infrastructure updates including buffer output capture

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