Add export_thread message action

[gonzaloserrano]

Adds a staff-only "Export thread" message action. It fetches the whole thread, renders it as Markdown and sends it to the requester as a .md file via DM. Useful for feeding thread content (e.g. specs) to an agent.

Behavior

• Works from the parent message or any reply; the thread root is resolved from thread_ts.
• Non-staff users get the existing "Not allowed" modal.
• Runs in background (Slack's 3s ack limit). Reads with the bot token and falls back to the admin token when the bot is not a member of the channel. Errors are reported to the requester via DM.
• Output: header (channel, link, message count, export time), then per message: author, UTC timestamp, text, and file links. User mentions, channel references, user groups, special mentions and links are converted from Slack mrkdwn to plain Markdown; HTML entities are unescaped.
• Records a thread.exported metric.

Slack app config needed before deploy

• Interactivity & Shortcuts: add a message shortcut with callback ID export_thread.
• Bot token scopes: files:write, im:write, users:read (reinstall the app if any are added).

Testing

• Unit tests for the mrkdwn → Markdown conversion and the rendered output (bot/export_thread_test.go).
• go test ./... and make lint pass.
• Not tested against a live workspace.

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https://claude.ai/code/session_01A6cUQyKYWYm11Eha7pWKMS

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