Retro board report posting

[sunba91-su]

Goal

Post formatted retrospective board reports to team channels.

Acceptance Criteria

• /retro board [--team T] - posts retro board to team channel
  • Formatted with emoji per column (Went Well, To Improve, Action Items)
  • Grouped by column, sorted, with user mentions
• /retro list [--team T] - lists recent retros with submission counts
• Posts to team's configured channel
• Permission: team lead or admin
• Handles: no retro found, retro still open (partial results)

Technical Notes

• Build on buildReport pattern from commands/standup.go
• Emoji prefixes per column
• Fetch team channel from store.GetTeam()

Dependencies

• Blocked by: Retrospection conversation DM flow

[sunba91-su]

Blocked by: #29