feat(board): design and scaffold GitHub Projects v2 board schema

[snipcodeit]

Summary

• Design GitHub Projects v2 board structure with 5 MGW pipeline custom fields
• Document complete field schema (Status → pipeline_stage, AI Agent State, Milestone, Phase, GSD Route)
• Scaffold board configuration reference for use by subsequent mgw:board command (Add mgw:board command — create, show, and configure project board #72)
• Provide machine-readable field schema in .mgw/board-schema.json and human-readable design in docs/BOARD-SCHEMA.md

Closes #71

Milestone Context

• Milestone: v2 — GitHub Projects Board Management
• Phase: 13 — Board Foundation & Field Schema
• Issue: 1 of 9 in milestone
• Dependencies: No dependencies. Unblocks: Add mgw:board command — create, show, and configure project board #72, Sync project.json milestones into board items on mgw:project init and extend #73, Auto-update board Status field on pipeline_stage transitions #74, Configure Board layout with pipeline-stage swimlanes for kanban view #77, Configure Table layout with sortable triage columns for team planning #78, Configure Roadmap layout with milestone-based timeline grouping #79

Changes

• docs/BOARD-SCHEMA.md — 394-line board design documentation covering all 5 custom fields, GraphQL mutation templates, project.json board key schema, and planned board views
• .mgw/board-schema.json — Machine-readable field definitions for mgw:board consumption (gitignored, local state)

Field Schema Summary

Field	Type	Options / Format
Status	Single Select	13 options — 1:1 mapping to all pipeline_stage values
AI Agent State	Text	planner:running, executor:running, verifier:running, idle, blocked
Milestone	Text	Milestone title string from project.json
Phase	Text	<number> — <name> format
GSD Route	Single Select	quick, quick --full, plan-phase, new-milestone

Test Plan

• Board schema captures all 13 pipeline_stage values in Status field options
• GSD Route options cover all 4 known routes
• project.json board key schema is documented (for mgw:board to write after creation)
• GraphQL mutation patterns are present for each field type
• Design is coherent and implementable in Add mgw:board command — create, show, and configure project board #72 (mgw:board command)
• .mgw/board-schema.json is valid JSON and contains all field definitions

GSD Artifacts

• Phase directory: .planning/phases/13-board-foundation-field-schema/
• Plan: 13-01-PLAN.md
• Summary: 13-SUMMARY.md
• Verification: 13-VERIFICATION.md