docs: add tool-integration checklist + stop hardcoding roster counts (#663)

Two related drift traps, both from hand-typed numbers/lists that no guard
watches:

1. CONTRIBUTING had no "how to add a tool" checklist, and its wording
   ("all output is gitignored") implied gitignoring was automatic — so
   tool contributors kept committing generated integrations/<tool>/ output.
2. The division set and agent/division counts were hardcoded in prose in
   several places and had already gone stale (CONTRIBUTING said "16" and
   omitted healthcare; EXECUTIVE-BRIEF said "9 divisions").

Changes:
- Add an "Adding a Tool Integration" checklist to CONTRIBUTING (discuss-first,
  reuse an existing `format`, the ~5-file touch list incl. the required
  .gitignore rule, run check-tools.sh). Harmonize the "committed build
  output" policy line to point at it.
- De-hardcode the division list in CONTRIBUTING — defer to divisions.json.
- Stop scattering roster counts: strategy/EXECUTIVE-BRIEF ("9 divisions") and
  check-agent-originality.sh ("184-agent library") drop the number entirely;
  README keeps a showcase stat but softens "232 across 16" to "230+ across
  every division" so it never becomes a lie as the roster grows.


Claude-Session: https://claude.ai/code/session_01WKnDRWM4izsB8WAXKszhsq

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

strategy/nexus-strategy.md

@@ -1103,7 +1103,7 @@ Use the NEXUS QA Feedback Loop Protocol format
 
 <div align="center">
 
-**🌐 NEXUS: 9 Divisions. 7 Phases. One Unified Strategy. 🌐**
+**🌐 NEXUS: All Divisions. 7 Phases. One Unified Strategy. 🌐**
 
 *From discovery to sustained operations — every agent knows their role, their timing, and their handoff.*