feat(status): add milestone health report with board URL and velocity

[snipcodeit]

Summary

• Extends mgw:status with a Milestone Health section showing completion ratio (done/total), velocity (issues/day), and blocked count
• Adds a board URL banner at the top of status output when board_url is configured in project.json
• Adds --board flag to open the project board directly in a browser via xdg-open
• Velocity is computed from .mgw/active/ and .mgw/completed/ file modification times — no new state fields required

Closes #83

Milestone Context

• Milestone: v2 — Team Collaboration & Lifecycle Orchestration
• Phase: 17 — Lifecycle Reporting & Roadmap Rendering
• Issue: 4 of 6 in milestone

Changes

commands/status.md

• argument-hint — added [--board] flag documentation
• parse_arguments — added OPEN_BOARD=false init and --board case
• detect_project — added BOARD_URL="" initialization (ensures variable always defined)
• load_project — added BOARD_URL extraction from project.json (board_url or board.url)
• open_board (new step) — opens board URL via xdg-open/open and exits; errors when no board configured
• compute_health (new step) — computes HEALTH_DONE, HEALTH_BLOCKED, HEALTH_VELOCITY from issue stages and .mgw/ file mtimes
• display_dashboard — added board URL banner (conditional) before header; added Milestone Health section after issue table
• json_output — added board_url top-level field and milestone.health sub-object
• success_criteria — added 7 new checklist items

Test Plan

• mgw:status with no board configured: no board URL line shown, Milestone Health section displays with velocity and blocked count
• mgw:status with board_url in project.json: board URL line appears before the ━━━ header
• mgw:status --board with board configured: opens URL via xdg-open, exits 0, does not render dashboard
• mgw:status --board with no board configured: prints error to stderr, exits 1
• mgw:status --json output includes board_url string and milestone.health object with done, total, blocked, velocity
• Velocity shows 0/day when no done issues, 1 (single) for one done issue, X.X/day for two or more
• Blocked count reflects issues in blocked pipeline stage
• mgw:status 2 (specific milestone number) works unchanged alongside new flags