Receive a Slack slash command, verify signature, and ack immediately.

Slack requires a response within 3 seconds. This endpoint verifies the request signature, parses the URL-encoded form payload, and returns an immediate acknowledgement. Any long-running work (task creation, etc.) should be dispatched asynchronously using response_url.

Reads SLACK_SIGNING_SECRET from environment for HMAC verification. The secret MUST be configured: when it is not, the endpoint is disabled and returns UNCONFIGURED_STATUS; only signed Slack requests are accepted. Returns 200 on success, 401 on bad/missing signature, 400 on parse error, UNCONFIGURED_STATUS when the endpoint is not configured.

Slash command form fields parsed: - command - the slash command (e.g. /bernstein) - text - text following the command - user_id - Slack user ID - channel_id - Slack channel ID - response_url - URL for delayed responses (up to 30 min) - trigger_id - trigger ID for opening modals