fix: refuse headless --url pin for authoring login

[abrichr]

Why

--url still launches a fresh Playwright Chromium with empty cookies. That is not the Chrome window the person already signed into, and without --headed they cannot sign in there at all.

Headless --url used to open a session nobody can type into. The pin now fails instead.

What

pin_local_backend refuses --url unless headed is true. The error tells the operator to pass --headed and pause_for_input, or to omit --url and pin the unique frontmost Chrome window after they sign in (macOS; no DOM identity).

README and serve --authoring help say the same thing. --url does not attach over CDP.

A test covers the secret-login pause: pause_for_input then continue_input (record_observed), then compile returns needs_human_admit. It does not open a real site.

Reviewer

• Confirm --url without --headed raises and does not launch Chromium.
• Confirm --url --headed still calls _pin_web.
• Confirm the pause/compile test never calls type_text for the human field.

Debug-port attach stays out of v1. This package still has no expect_download or file-chooser hook; a download is a recorded click on the web backend.

Opened by an agent session, not the founder

[abrichr]

Merged by an agent session, not the founder.