Retrospection conversation DM flow

[sunba91-su]

Goal

Implement the conversational DM collection flow for team retrospectives, similar to daily standups but with retro-specific questions and structure.

Acceptance Criteria

• Register commands under /standup namespace:
  • /standup retro start — starts a retro session for today
  • /standup retro add <column> <text> — adds an item to a retro column
  • /standup retro submit — finishes and saves retro
  • /standup retro cancel — aborts retro (deletes partial)
• Or create a separate /retro command namespace (preferred):
  • /retro start — starts a retro session
  • /retro add <text> — adds to current column (DM walks through columns one at a time)
  • /retro submit — finishes
  • /retro cancel — aborts
• DM conversation flow (reuse or extend convstate):
  • Bot: "Starting retro for team X. Column 1: What went well? (reply with your item)"
  • User replies
  • Bot: "Column 2: What could be improved?"
  • ...repeat for each column
  • Bot: "✅ Retro submitted! Thanks."
• Permission: team member or lead (same as standup submit)
• Multi-team support: --team flag for team selection
• Duplicate prevention: one retro per team per day (re-open allowed? optional)
• Update internal/commands/help.go with new commands

Technical Notes

• Reuse convstate.Manager — add a Type field to Conversation to distinguish standup vs retro
• Or create a separate retro conversation manager — evaluate tradeoffs
• Retro columns come from the session (created at session start via defaults or team config)
• Store items as they are added (immediate persistence, not just in-memory)

Dependencies

• Blocked by: Retrospection data model and store

[sunba91-su]

Blocked by: #26