[previewctl] Add error logging in install-context command

[geropl]

Problem

When previewctl install-context fails, the error message only shows exit status 1 without any context about what went wrong:

+ previewctl install-context --branch workspace-637d043885 --log-level debug --timeout 10m
Error: exit status 1
...
time="2026-01-12 13:32:49" level=fatal msg="command failed." err="exit status 1"

Root Cause

The failure happens before any logging occurs, likely in PreRunE when generating the SSH key. The GenerateSSHPrivateKey() function:

1. Doesn't create the parent directory if it doesn't exist (e.g., /github/home/.ssh/)
2. Uses cmd.Run() which discards all output, so ssh-keygen failures are silent

Solution

1. Fix SSH key generation (preview.go)

• Create the parent directory with os.MkdirAll() before running ssh-keygen
• Capture and include ssh-keygen output in error messages

2. Add error logging in install function (install_context.go)

• Log errors with context at each failure point:
  • failed to get preview name
  • failed to create preview config
  • failed to install context

This complements PR #21246 which added error logging in installVMSSHKeys().