fix(slack): harden M4 edit and regenerate flows (#22)

[yashoza19]

Follow-up to #20 — addresses the gaps tracked in #22 so M4 edit/regenerate is usable in production.

• Edit modal: one input per epic (blank = remove) to stay within Slack’s 10 input-block limit; up to 8 epics per page with pagination hints
• Ledger hygiene: new src/status/db/edit.py — only supersede changed/dropped rows; unchanged entries stay current with audit chain intact
• Slack UX: edit and regenerate update the original DM via chat.update (channel/ts in modal metadata); no duplicate draft messages
• Regenerate: runs collect + drafter in a background thread after ack(); persists participation.regenerated and regenerate_reason; user notes passed to drafter via regeneration_notes (not collection flags)
• Other: Slack user authorization on edit/regenerate, unticketed prefill from flag/entry, aggregate leadership ask on first epic only

Test plan

• pytest tests/test_slack_edit.py tests/test_slack_regenerate.py tests/test_slack_confirm.py tests/test_draft_persist.py
• Open edit modal on a draft with 5+ epics — modal opens without Slack API error
• Edit one outcome → original message updates in place; source='drafted_edited' on changed row
• Clear an epic field → entry dropped from draft
• Regenerate with reason → ephemeral “may take a minute” message, then original DM refreshes; participation.regenerated = true
  Closes M4 follow-up: harden Slack edit/regenerate flows (PR #20) #22

[exe-prow-github-app]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: yashoza19

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment