Open device verification in the default browser with a headless fallback

[adrianwebb]

Objective

When trsd auth login receives verification_uri_complete, attempt to open it in the operator's default browser and retain a copyable URL for headless or remote use.

Acceptance

• The complete verification URL, including the user code, is passed to the platform opener without shell interpolation.
• Success output tells the operator to follow the browser instructions.
• A copyable complete URL is always printed for headless/remote authorization.
• Opener failure does not fail or delay device polling.
• JSON command output remains machine-readable on stdout.
• Linux, macOS, and Windows launch commands are covered through an injected boundary.

[adrianwebb]

Delivered and published as 0.13.0-rc.27 from staging (PR #65). Release verification passed 44/44. The CLI now attempts the platform default browser with the complete device verification URL, emits only a concise follow-browser message on success, and always retains the copyable URL for headless/remote authorization. JSON stdout remains stable and guidance stays on stderr.