fix(slackbot): read routed DM metadata payloads

[samuelclay]

Summary

• Request include_all_metadata when Slackbot reads routed DM root messages.
• This lets the approval handler recover the original channel/thread payload instead of seeing only event_type and falling through to a new agent handoff.
• Add a regression assertion covering the Slack metadata payload flag.

Context

PR #21 wrote route information into Slack message metadata and expected approval replies like post back to copy the latest successful Watch Agent result back to the original thread. In production, Slack returned only the metadata event_type unless include_all_metadata=true was set on conversations.replies, so the handler could not parse the route payload and ignored the approval.

Observed on routed DM thread D0B7DPBT69M / 1783436525.851069: the reply ok post that back was handed to a new Watch Agent turn instead of posting to original thread C0BEAU4B66T / 1783436286.363169.

Test plan

• bun test src/slack/thread-routing.test.ts src/index.test.ts src/centaur/handoff.test.ts src/centaur/final-delivery.test.ts
• bun run check:types
• bun test src/centaur/handoff.test.ts src/slack/normalize.test.ts src/slack/home.test.ts src/index.test.ts src/centaur/final-delivery.test.ts
• docker build -t centaur-slackbot:latest -f services/slackbot/Dockerfile .

Live diagnosis also confirmed that conversations.replies without include_all_metadata returns only { event_type }, while the same call with include_all_metadata=true returns the full event_payload containing the source channel and thread.