feat(board): configure Board layout view with pipeline-stage swimlanes

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Summary

• Add mgw:board views subcommand to create GitHub Projects v2 layout views (kanban, table, roadmap)
• Create Kanban board view grouped by Status field — each pipeline stage becomes a swimlane
• Document view configuration in docs/BOARD-SCHEMA.md with field schema, option values, and view specs
• Output step-by-step user instructions for setting group-by in GitHub UI (API limitation)

Closes #77

Milestone Context

• Milestone: v2 — GitHub Projects Board Management
• Phase: 15 — Multi-Layout Views
• Issue: 7 of 9 in milestone

Changes

• Added: .claude/commands/mgw/board.md — new views subcommand (kanban/table/roadmap layouts)
• Added: docs/BOARD-SCHEMA.md — documents custom fields, Status options, GSD route options, and three intended views

Implementation Notes

GitHub's Projects v2 GraphQL API supports createProjectV2View but does not expose a mutation for setting the "Group by" field on a view. The views kanban subcommand:

1. Creates the BOARD_LAYOUT view via GraphQL
2. Stores the view ID in .mgw/project.json under project.project_board.views
3. Outputs numbered instructions for the user to open the board and set "Group by: Status"

Test Plan

• mgw:board views kanban creates a BOARD_LAYOUT view in the project
• View ID is stored in .mgw/project.json under project.project_board.views.kanban
• User receives clear numbered instructions to set Status grouping in GitHub UI
• mgw:board views table and mgw:board views roadmap work analogously
• mgw:board views (no subcommand) prints usage message
• mgw:board show displays configured views with name, layout, and ID
• docs/BOARD-SCHEMA.md documents all three intended views and their configuration

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Work included in main via integration merge from milestone-1 branch.