Add SSH connection detection and fallback URL output

[Copilot]

The wp admin command fails on SSH connections with "xdg-open: not found" because it attempts to open a browser on the remote server.

Changes

SSH Detection

• Check SSH_CONNECTION, SSH_CLIENT, and SSH_TTY environment variables
• Output URL directly when SSH detected instead of attempting browser open

Error Handling

• Replace passthru() with exec() to capture return codes
• Fallback to URL output when browser command fails (e.g., xdg-open not installed)

Example behavior on SSH:

$ wp admin
Detected SSH connection. Please open this URL in your local browser:
http://example.com/wp-admin/

Local behavior unchanged - still opens browser automatically on macOS/Windows/Linux.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Admin with SSH (remote) connections</issue_title>
<issue_description>Does this package work with ssh (remote) connections? I just tried it and got a "xdg-open: not found" error.</issue_description>

Comments on the Issue (you are @copilot in this section)

@schlessera @dicetomato No, that cannot work. How would the remote server be able to control your browser? @schlessera That being said, it could theoretically work if we bypass the normal SSH execution flow, and just return data from the site back to the admin command. Not sure whether that is easily doable, given the current architecture, though...

• Fixes Admin with SSH (remote) connections #12

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