#117 · Clarify default !rollup scope in help text

Context

!rollup with no flags returns the community-wide weekly digest. A user typing !rollup in #general reasonably expects to see what THEIR group discussed and gets cross-community stats instead. The help text at apps/signal-bot/src/src/bot/command-handler.ts:1933 says "!rollup - Weekly community digest" — accurate-ish but doesn't surface the scope distinction up front.

Acceptance criteria

• [ ] Update !help Scoreboard / Community section so the rollup entry reads roughly:

  !rollup           - Community-wide weekly digest
  !rollup -c        - This group's weekly digest
  !rollup -d N      - Last N days
  !rollup -m N      - Last N months
  !rollup --full    - Outline doc (admin or with -c)

• [ ] Update the docstring above handleRollup to match
• [ ] Optional: when !rollup runs WITHOUT -c in a group context, append a one-liner to the output: _Tip: \!rollup -c\ for just this group's digest._

Technical hints

• Help text lives at command-handler.ts:1932-1937
• Docstring: command-handler.ts:30952-30962
• Don't break the existing format — match the indentation and bullet style of surrounding entries

--- *XS-tier (25 pts + 40 = 65). Pure text change, perfect first-PR target.*