Slack thread replies rework the draft

[farzanmrz]

The correction machinery exists for email (api/email/inbound: Svix-verified reply → draft correction → revision call → re-delivery flagged "Revised draft"). This issue gives Slack the same loop:

• Reporter replies in the draft message's thread with rework instructions ("shorter", "lead with the quote")
• Slack Events subscription receives the reply; thread → draft mapping mirrors the plus-addressed email decoder pattern in lib/notify/email.ts — encode/decode in one file so they cannot drift
• Reply text feeds the same revision path (nano, after roadmap B — extractor→drafter contract: beat filtration, translation, synthesis actually govern the draft #73's unification) → re-delivered as Revised draft in the same thread
• "Yes, post" stays a button (exists via the interactions route), not a parsed reply

Scope discipline: no free-form chat with the agent — one reply = one revision instruction.

Depends on #74.

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Tooling note (2026-07-26): Same tooling picture as #74 — no Slack app-dev CLI installed; claude.ai Slack connector MCP is available for in-channel verification of the reply→revision loop; vercel:chat-sdk covers Events API/webhook patterns but the repo convention is raw fetch in lib/slack (mirror api/slack/interactions' raw-body HMAC + 3s-ack pattern for the Events subscription).

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Docs grounding + planning inputs (2026-07-26): Use the slack-docs agent (~/.claude/agents/slack-docs.md) for all Slack API grounding. Three findings that shape this issue's plan:

1. Scope expansion → re-consent: reading thread replies needs channels:history-family scopes beyond the current grant — adding them to the per-agent app likely forces a re-install/re-consent for every existing install; plan the migration UX.
2. The new Events endpoint must pass the url_verification challenge handshake (echo challenge within 3s) before Slack delivers anything — same raw-body HMAC verification as the interactions route.
3. thread_ts semantics are not fully specified on the message.channels event page — cross-check the message_replied subtype reference before writing thread-detection/dedupe logic. Events API retries make idempotency mandatory.

Optional development aid: claude plugin install slack connects Slack's hosted MCP (mcp.slack.com) to Claude Code sessions for live in-channel verification during QC.