Add per-stage spinners to mgw:run pipeline steps

## Description
Wrap each pipeline step (validate, triage, create-worktree, execute-gsd, create-pr) with a spinner showing stage name and elapsed time. Use ora or similar.

## Acceptance Criteria
- [ ] Add per-stage spinners to mgw:run pipeline steps

## GSD Route
quick

## Phase Context
Phase 28: Pipeline Progress Indicators and Live Status of v4 — Interactive CLI & TUI

## Depends on
_none_