podstack v0.9.0

Plan-first workflow with a resumable progress checkpoint (like Claude Code):

• The agent writes the plan to .podstack/plans/<name>.md before building.
• A new todo tool tracks the task list and checkpoints it to .podstack/progress.json, so progress survives across sessions — resume and it picks up where it left off.
• Tasks are marked in_progress/completed as the build proceeds.