Fix help text, enable --tag-turns for BQ, and fix --env loading

- --limit: clarify it evaluates the N most recent sessions
- --trajectory-samples: remove misleading "Requires BQ credentials"
- --tag-turns: remove conversations-file restriction, enable for BQ path
  by passing tag_turns through run_evaluation with concurrent tagging
- --env=path syntax now works in shell script (not just --env path)
- Error message now says 'export VAR=...' and mentions --env flag

Author: evekhm