Creates an evidence directory under .evidence/<featureName>/<timestamp>/ in the current project. Returns a sessionId to pass to request. Call finish_evidence_session when done.

Makes an HTTP request and logs the full request/response pair (headers, body, status, timing) as evidence. Sensitive headers (authorization, cookie, api-key, etc.) are redacted before being written to disk. url may be relative to the session's baseUrl.

Writes requests.json (all request/response pairs) and manifest.json (summary + failure count), and returns the evidence folder path. Always call this at the end of a verification run.