Start (or fetch) the browser session of a user-triggered playbook action

Starts the browser session for a user-triggered form-filling playbook action and returns `{ data: { sessionId, pairUrl, controlUrl, status, ... } }`. The caller opens the pairUrl in a compatible browser with the CloudRaker extension installed; once paired, the playbook agent drives that tab to fill the form. Idempotent: while a session is open the same session (and pairUrl) is returned. Authorization: `space:contribute` (org admins bypass); missing → 403. Returns 409 when dependencies are unmet or another session open, 422 not a user-triggered action, 502 browser session unavailable.