feat(skills): integrate start-sprint into sprint-start skill

Move start-sprint.sh into .claude/skills/sprint-start/scripts/ (was
standalone at parent directory level). Sprint-start Case B now calls
the script automatically after activating a sprint — creates ephemeral
worktrees and opens Ptyxis tabs for each active team.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.claude/settings.json

@@ -64,6 +64,8 @@
       "Bash(list *)",
       "Bash(tree *)",
       "Bash(sed -n *)",
+      "Bash(.claude/skills/sprint-start/scripts/start-sprint.sh *)",
+      "Bash(.claude/skills/sprint-start/scripts/sprint-teardown.sh *)",
 
       "Skill(git-commit)",
       "Skill(sprint-start)",