feat(board): surface AI Agent State field during GSD execution

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Summary

• Add update_board_agent_state utility to board-sync.md — updates the "AI Agent State" TEXT field on board items via GitHub Projects v2 GraphQL API
• Hook into run.md at planner/executor/verifier spawn points for both quick and plan-phase routes
• Shows current agent activity in GitHub Projects board (e.g., "Planning phase 14", "Executing phase 14")
• Non-blocking: API failures never interrupt pipeline execution

Closes #75

Milestone Context

• Milestone: v2 — GitHub Projects Board Management
• Phase: 14 — Pipeline State Sync
• Issue: 5 of 9 in milestone

Changes

• Updated: .claude/commands/mgw/workflows/board-sync.md — added update_board_agent_state function alongside existing update_board_status
• Updated: .claude/commands/mgw/run.md — inline update_board_agent_state definition + calls before each GSD agent spawn and clear after PR creation

AI Agent State Update Points

Trigger	Value
Before gsd-planner (quick)	"Planning"
Before gsd-executor (quick)	"Executing"
Before gsd-verifier (quick)	"Verifying"
Before gsd-planner (plan-phase, phase N)	"Planning phase N"
Before gsd-executor (plan-phase, phase N)	"Executing phase N"
Before gsd-verifier (plan-phase, phase N)	"Verifying phase N"
After PR created	"" (clear)

Test Plan

• During planning, board shows "Planning phase N"
• During execution, board shows "Executing phase N"
• During verification, board shows "Verifying phase N"
• After PR creation, field is cleared to empty string
• If board not configured (no node_id), all calls are silent no-ops
• If issue has no board_item_id, all calls are silent no-ops
• If ai_agent_state field not in board schema, all calls are silent no-ops
• GraphQL API errors do not halt pipeline execution

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