feat: add /mgw:status project dashboard command

[snipcodeit]

Summary

• Adds /mgw:status [milestone_number] [--json] command for project-wide visibility into milestone progress, per-issue pipeline stages, open PRs, and next milestone preview
• Implements graceful fallback to GitHub-only mode when no project.json exists (lists open issues and PRs from the API)
• Supports --json flag for machine-readable output suitable for scripting and automation
• Updates help text and README to include the new command in the Query section

Closes #37

Milestone Context

• Milestone: v1 — Pipeline Intelligence
• Phase: 8 — Query Layer
• Issue: 3 of 4 in milestone

Changes

• commands/status.md — New slash command definition with full dashboard rendering logic
• .claude/commands/mgw/status.md — Mirror of the above for deployed slash commands
• commands/help.md — Added /mgw:status under new "Query" section
• .claude/commands/mgw/help.md — Mirror update matching commands/help.md
• README.md — Added status command to commands table, project structure listing, and verify output

Test Plan

• Run /mgw:status in a repo with project.json — verify milestone progress bar, issue table, and PR list render correctly
• Run /mgw:status in a repo without project.json — verify graceful fallback shows GitHub-only issue/PR listing
• Run /mgw:status --json — verify structured JSON output with correct schema
• Run /mgw:status 1 — verify non-current milestone can be viewed
• Run /mgw:help — verify status command appears in the Query section
• Verify commands/status.md and .claude/commands/mgw/status.md are identical
• Verify commands/help.md and .claude/commands/mgw/help.md are identical