feat: comment-aware pipeline for MGW

[snipcodeit]

Summary

• Add comment tracking to MGW pipeline so it detects new comments posted on issues between triage and execution
• Classify comments as material (scope change), informational (no impact), or blocking (halt pipeline) using a general-purpose agent
• Add pre-flight comment check in run.md that gates GSD execution on comment review
• Create standalone /mgw:review command for on-demand comment classification

Closes #36

Milestone Context

• Milestone: v1 — Pipeline Intelligence
• Phase: 7 — Pipeline Fixes
• Issue: 2 of 4 in milestone

Changes

• State schema (workflows/state.md): Added triage.last_comment_count and triage.last_comment_at fields to issue state schema; added blocked as valid pipeline stage; documented comment tracking lifecycle, classification rules, and drift detection
• Pipeline orchestrator (commands/run.md, .claude/commands/mgw/run.md): Added preflight_comment_check step between worktree creation and triage comment posting; spawns classification agent when new comments detected; handles material/informational/blocking responses
• Triage (commands/issue.md, .claude/commands/mgw/issue.md): Added comment snapshot capture in fetch_issue step; populates last_comment_count and last_comment_at in write_state step
• Sync (commands/sync.md, .claude/commands/mgw/sync.md): Added comment delta check in check_each step; reports unreviewed comments in sync summary
• Review command (commands/review.md, .claude/commands/mgw/review.md): New standalone command for on-demand comment review with interactive action selection
• GitHub patterns (workflows/github.md): Added comment count, recent comments, and last comment timestamp CLI patterns
• GSD patterns (workflows/gsd.md): Added comment classification agent spawn template
• Validation (workflows/validation.md): Added review.md to delegation boundary table

Test Plan

• Verify triage.last_comment_count and triage.last_comment_at are populated when /mgw:issue runs triage on an issue with existing comments
• Verify /mgw:run pre-flight check correctly detects new comments when count exceeds stored value
• Verify pre-flight check skips gracefully when state file has no comment tracking fields (backward compatibility)
• Verify blocking classification sets pipeline_stage to blocked and halts execution
• Verify material classification enriches planner context with new requirements
• Verify informational classification logs and continues without blocking
• Verify /mgw:sync reports comment drift in sync summary
• Verify /mgw:review works standalone for manual comment triage
• Verify both commands/ and .claude/commands/mgw/ mirrors are in sync