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/project-hygiene/hooks/cleanup.sh

@@ -10,6 +10,17 @@ PREFIX="[project-hygiene]"
 # Read tool input from stdin (discard - we don't need it for cleanup)
 cat > /dev/null
 
+# Cooldown - skip if ran in last 60 seconds
+# Using $PPID ties cooldown to the Claude session
+LAST_RUN_FILE="/tmp/project-hygiene-${PPID}-last-run"
+if [[ -f "$LAST_RUN_FILE" ]]; then
+    LAST_RUN=$(cat "$LAST_RUN_FILE")
+    NOW=$(date +%s)
+    if (( NOW - LAST_RUN < 60 )); then
+        exit 0
+    fi
+fi
+
 PROJECT_ROOT="${PROJECT_ROOT:-.}"
 DELETED_COUNT=0
 
@@ -25,4 +36,7 @@ if [[ $DELETED_COUNT -gt 0 ]]; then
     echo "$PREFIX Cleaned $DELETED_COUNT temp files"
 fi
 
+# Record this run for cooldown
+date +%s > "$LAST_RUN_FILE"
+
 exit 0