feat(assign): add mgw:assign command — claim issues and update board assignment

[snipcodeit]

Summary

• Adds mgw:assign <issue-number> [username] command to claim GitHub issues for a user (defaults to @me)
• Assigns via gh issue edit --add-assignee, updates .mgw/active/ state, and emits board URL confirmation
• Creates a minimal .mgw/active/ state entry on-demand if the issue has not yet been triaged — enabling assignment at any point in the pipeline
• Relies on GitHub Projects v2's built-in issue-assignee propagation to board items, avoiding a fragile GraphQL mutation on a non-standard field

Closes #80

Milestone Context

• Milestone: v2 — Team Collaboration & Lifecycle Orchestration
• Phase: 16 — Team Awareness & Assignment
• Issue: 1 of 6 in milestone

Changes

commands/assign.md (new)

• parse_args — splits $ARGUMENTS into ISSUE_NUMBER + optional USERNAME, validates numeric
• validate_and_load — initializes .mgw/ structure, loads existing state file or flags for creation
• resolve_user — defaults to @me via gh api user; validates named user via gh api users/$USERNAME
• fetch_issue — fetches issue metadata; detects if already assigned (idempotent path)
• assign_github — calls gh issue edit --add-assignee; skips if already assigned
• update_state — writes assignee to existing state file, or creates minimal pipeline_stage: new entry
• check_board — reads project.json for board URL; emits URL for confirmation (no GraphQL mutation needed)
• confirm — prints formatted assignment confirmation table

~/.claude/commands/mgw/assign.md (deployed)

• Identical copy of commands/assign.md deployed to user's Claude commands directory

Test Plan

• mgw:assign 80 — assigns issue Add mgw:assign command — claim issues and update board assignment #80 to authenticated user, state file updated
• mgw:assign 80 snipcodeit — assigns to named user, confirms assignment
• Re-run mgw:assign 80 — idempotent path: "already assigned" message, no error
• Run on issue with no existing .mgw/active/ state — minimal state entry created
• With board configured: board URL appears in confirmation output
• Without board configured: board section absent from output
• Invalid issue number (non-numeric) — error message, no side effects
• Non-existent GitHub user — error message, no assignment attempted