perf(hooks): improve hook efficiency with early exits and cooldowns

Sprint 8 - Hook Efficiency Quick Wins (v5.5.0)

- Remove viz-platform SessionStart hook (zero value, just echoed "loaded")
- Add early git check to git-flow branch-check.sh (skip JSON parsing for non-git commands)
- Add early git check to git-flow commit-msg-check.sh (skip Python spawn for non-commit commands)
- Add 60-second cooldown to project-hygiene cleanup.sh (reduce find operations)

Closes #321, #322, #323, #324

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/viz-platform/hooks/hooks.json

@@ -1,10 +1,3 @@
 {
-  "hooks": [
-    {
-      "event": "SessionStart",
-      "type": "command",
-      "command": "echo 'viz-platform plugin loaded'",
-      "timeout": 5000
-    }
-  ]
+  "hooks": []
 }