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/EXECUTIVE-BRIEF.md

@@ -6,7 +6,7 @@
 
 ## 1. SITUATION OVERVIEW
 
-The Agency comprises specialized AI agents across 9 divisions — engineering, design, marketing, product, project management, testing, support, spatial computing, and specialized operations. Individually, each agent delivers expert-level output. **Without coordination, they produce conflicting decisions, duplicated effort, and quality gaps at handoff boundaries.** NEXUS transforms this collection into an orchestrated intelligence network with defined pipelines, quality gates, and measurable outcomes.
+The Agency comprises specialized AI agents across every division — engineering, design, marketing, security, GIS, product, testing, and more. Individually, each agent delivers expert-level output. **Without coordination, they produce conflicting decisions, duplicated effort, and quality gaps at handoff boundaries.** NEXUS transforms this collection into an orchestrated intelligence network with defined pipelines, quality gates, and measurable outcomes.
 
 ## 2. KEY FINDINGS
 
@@ -92,4 +92,4 @@ strategy/
 
 ---
 
-*NEXUS: 9 Divisions. 7 Phases. One Unified Strategy.*
+*NEXUS: All Divisions. 7 Phases. One Unified Strategy.*