review: gate Pi on 0.65.1 and keep --target's --prompt out of argv

Pi before 0.65.1 printed plain text instead of JSONL for a prompt piped on stdin with --mode json, which the review parser reports as a failed run. ReviewerTemplate gains an optional Preflight that runs before the command is built; Pi's checks pi --version and refuses an older release with an upgrade message. A version it cannot run or parse is let through.

entire review --target re-runs review in the target worktree and forwarded --prompt as --prompt=<text> in the child's argv. It now passes it in ENTIRE_REVIEW_TARGET_PROMPT, which the child honors only as a target child (ENTIRE_REVIEW_FINDINGS_WORKTREE set) and unsets so reviewer agents don't inherit it.