chore(plan-rollout): rename docs/SYSTEM-MD.md → docs/SYSTEM_MD.md

Match the SCREAMING_SNAKE_CASE convention used by other topic docs in
docs/ (ADDING_A_HOST.md, OPENCLAW.md, REMOTE_BROWSER_ACCESS.md,
ON_THE_LOC_CONTROVERSY.md). The hyphenated form was carried over from
closed PR garrytan#1417 and matched no existing convention in this directory.

No content changes — pure rename.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mastermanas805