feat(board): sync project.json milestones into board items on project init/extend

[snipcodeit]

Summary

• Auto-populate board with issues on mgw:project init and extend
• Sets Milestone, Phase, and GSD Route fields on each board item
• Stores board_item_id in project.json for future updates
• Non-blocking: board sync failure doesn't block project initialization

Closes #73

Milestone Context

• Milestone: v2 — GitHub Projects Board Management
• Phase: 13 — Board Foundation & Field Schema
• Issue: 3 of 9 in milestone

Changes

• Updated: commands/project.md (runtime: ~/.claude/commands/mgw/project.md) — added sync_milestone_to_board step between create_project_board and write_project_json

Implementation Details

The new sync_milestone_to_board step:

1. Reads project.project_board.node_id and project.project_board.fields from project.json
2. If board is configured with custom fields, iterates ISSUE_RECORDS (populated in create_issues)
3. For each issue: resolves the GitHub node ID via GraphQL, calls addProjectV2ItemById, then calls updateProjectV2ItemFieldValue for Milestone (TEXT), Phase (TEXT), and GSD Route (SINGLE_SELECT) fields
4. Collects board item IDs into ITEM_ID_MAP bash array
5. After all issues processed, write_project_json decodes ITEM_ID_MAP into ITEM_ID_MAP_DICT and writes board_item_id per issue record

Skip conditions (silent):

• PROJECT_NUMBER is empty (board not created)
• BOARD_NODE_ID is empty (board exists but fields not configured via /mgw:board create)

Error handling: each GraphQL call uses || BOARD_SYNC_WARNINGS+=(...) to accumulate warnings. All warnings are printed after the loop. Pipeline never halts on board sync failure.

Test Plan

• After mgw:project init, board items exist for all milestone issues
• Milestone, Phase, GSD Route fields are set on each item
• board_item_id stored in project.json for each issue
• If board not configured (no node_id), sync is skipped silently with NOTE message
• If board created but fields not configured, sync is skipped with NOTE message
• If a single GraphQL call fails, remaining issues continue processing
• Extend mode: only new milestone issues are synced (existing board items unchanged)