feat(board): add mgw:board command for project board management

[snipcodeit]

Summary

• Add new /mgw:board command with create, show, and configure subcommands
• Integrates with GitHub Projects v2 GraphQL API to manage board creation and field schema
• Uses board schema from Design and scaffold GitHub Projects v2 board with MGW custom fields #71 (docs/BOARD-SCHEMA.md) — 5 custom fields with pipeline_stage mapping
• Stores board node_id, field IDs, and option IDs in project.json for downstream commands

Closes #72

Milestone Context

• Milestone: v2 — GitHub Projects Board Management
• Phase: 13 — Board Foundation & Field Schema
• Issue: 2 of 9 in milestone
• Depends on: Design and scaffold GitHub Projects v2 board with MGW custom fields #71 (DONE — PR feat(board): design and scaffold GitHub Projects v2 board schema #86 merged)

Changes

• New: .claude/commands/mgw/board.md — full board command implementation

mgw:board create (idempotent)

• Resolves owner node ID via GraphQL (user + org fallback)
• Creates GitHub Projects v2 board via createProjectV2 mutation
• Creates 5 custom fields: Status (13 SINGLE_SELECT options), AI Agent State (TEXT), Milestone (TEXT), Phase (TEXT), GSD Route (4 SINGLE_SELECT options)
• Status options map 1:1 to all pipeline_stage values
• Stores board number, URL, node_id, field IDs, and option IDs in project.json under project.project_board.fields
• Exits cleanly if board already configured (no duplicate creation)

mgw:board show

• Displays board URL, node ID, and field IDs with types
• Fetches live board items from GitHub via GraphQL
• Groups items by Status field value in pipeline order
• Handles empty boards with helpful next-step message
• Falls back to org query if user query fails

mgw:board configure

• Fetches current field state from GitHub
• Compares all fields against canonical schema from docs/BOARD-SCHEMA.md
• Reports missing Status options, GSD Route options, and text fields
• Explains GitHub Projects v2 API limitation on modifying existing single-select options

Test Plan

• /mgw:board create creates board and stores in project.json (run once, verify project.json updated)
• /mgw:board create (run again) exits cleanly with existing board URL — idempotency confirmed
• /mgw:board show displays board URL, field list, and items grouped by Status
• /mgw:board show on empty board shows "No items" with helpful next-step
• /mgw:board configure fetches field state and reports schema comparison
• GraphQL mutations match patterns from docs/BOARD-SCHEMA.md (Design and scaffold GitHub Projects v2 board with MGW custom fields #71)
• project.json updated with node_id, field IDs, option IDs under project.project_board.fields
• Error if no project.json (clear message to run /mgw:project first)
• Error if board not configured for show/configure (clear message to run create)