Support nested agent directories and add game-development category

- Add game-development to AGENT_DIRS in convert.sh, lint-agents.sh,
  install.sh, and the CI workflow
- Remove maxdepth 1 from find commands so nested subdirectories
  (e.g., game-development/unity/) are discovered
- Update CI git diff globs to **/*.md for recursive matching
- Remove strategy from lint AGENT_DIRS (playbooks, not agents)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/lint-agents.sh

@@ -13,6 +13,7 @@ set -euo pipefail
 AGENT_DIRS=(
   design
   engineering
+  game-development
   marketing
   paid-media
   product
@@ -21,7 +22,6 @@ AGENT_DIRS=(
   support
   spatial-computing
   specialized
-  strategy
 )
 
 REQUIRED_FRONTMATTER=("name" "description" "color")
@@ -87,7 +87,7 @@ else
     if [[ -d "$dir" ]]; then
       while IFS= read -r f; do
         files+=("$f")
-      done < <(find "$dir" -maxdepth 1 -name "*.md" -type f | sort)
+      done < <(find "$dir" -name "*.md" -type f | sort)
     fi
   done
 fi