docs: remove --agent-file flag; direct provider env vars; no harness key; rename steps→tasks

Agent-Logs-Url: https://github.com/github/gh-aw/sessions/e3669f00-f88f-43f3-b828-20c156761234

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

Author: Copilot