Add --dry-run flag to mgw:project extend and init flow

## Description
Display full template output (milestone names, issues, dependency graph) without creating GitHub resources. Print summary table instead of making API calls.

## Acceptance Criteria
- [ ] Add --dry-run flag to mgw:project extend and init flow

## GSD Route
quick

## Phase Context
Phase 31: Dry-run Mode and Execution Preview of v4 — Standalone and CI Usage

## Depends on
_none_