refactor(board): renumber milestones relative to epics and collapse icebox

Changes:
- Milestones now use relative numbering within each epic (01, 02, 03...)
  instead of global numbers (38, 54, etc.)
- Icebox section in board README is now collapsed by default
- Updated all story scope fields to reference new milestone numbers
- Fixed octal interpretation bug in _next-milestone function

This makes it easier to understand the relative sequence of milestones
within each epic without needing to know the global ordering.

Author: rupurt

.justfiles/board.just

@@ -112,8 +112,9 @@ generate:
     fi
     echo ""
 
-    # Icebox stories
-    echo "## Icebox"
+    # Icebox stories (collapsed by default)
+    echo "<details>"
+    echo "<summary><h2>Icebox</h2></summary>"
     echo ""
     if [[ -d stages/icebox/stories ]] && ls stages/icebox/stories/*.md &>/dev/null 2>&1; then
         echo "| Story | Type | Priority | Scope |"
@@ -130,6 +131,8 @@ generate:
         echo "*No stories in icebox*"
     fi
     echo ""
+    echo "</details>"
+    echo ""
 
     # Epics section - group milestones under each epic
     echo "## Epics"