fix: strip Slack approval buttons immediately on click

[figitaki]

Summary

• Slack interactive prompts kept their Approve/Deny buttons clickable for the entire duration of the underlying tool execution. The handler returned 200 within Slack's 3-second deadline (packages/worker/src/routes/slack-events.ts:659), which dropped Slack's spinner, but the message wasn't actually updated until the DO finished executeActionAndSend (session-agent.ts:5836) and only then called chat.update. On slow actions that gap was many seconds, during which users could re-click Approve.
• Fix: before scheduling the DO call, POST to the payload's response_url with replace_original: true and the original blocks minus the actions block, plus a "⏳ Processing…" context line. The DO's existing final-state chat.update still runs at the end and overwrites with "✅ Approved by …" / "❌ Denied by …".
• Slack-only change. No ChannelTransport interface change. Wrapped in try/catch — if response_url fails the old behavior is preserved.

Test plan

• pnpm typecheck passes (verified locally)
• Deploy to dev: ENVIRONMENT=dev make deploy-worker
• Trigger an approval-gated action in Slack, click Approve — buttons should disappear within ~500ms and be replaced by "⏳ Processing…", then by "✅ Approved by …" once the action completes
• Repeat with Deny — same intermediate state, then "❌ Denied by …"
• Try a slow approval (real third-party API call) and confirm buttons stay gone for the entire processing window
• Check make logs-cloudflare for [Slack Interactive] response_url update failed warnings — should be zero

[figitaki]

We really need zod and/or the slack sdk. Too many unsafe casts throughout the codebase.