feat(slack-notify): add bot-token path with thread + update support

[vklimontovich]

Summary

• Adds bot-token path to the composite slack-notify action: when bot_token + channel are set, the action posts via chat.postMessage (or chat.update when update_ts is set) and outputs ts.
• Adds thread_ts input for posting threaded replies.
• Adds ts and channel outputs so callers can thread or edit later.
• Webhook path stays as backward-compatible fallback (with a warning if thread_ts / update_ts are passed — webhooks can't thread).
• Reusable workflow (.github/workflows/slack-notify.yml) surfaces the same new inputs/outputs and pulls the new DEPLOY_BOT_SLACK_TOKEN org secret directly (with CI_SLACK_WEBHOOK retained as fallback).

Required secrets

• DEPLOY_BOT_SLACK_TOKEN — already created at org level (xoxb-... from the deploy_bot Slack app, scope: chat:write).

Merge order

This PR is the first of three coordinated changes; the downstream PRs depend on it. After merging:

• Cut a new release tag if you want downstream callers to pin (optional — the downstream PRs currently reference @main).
• Then merge:
  1. jitsucom/jitsu#<PR> — services.yaml uses the reusable workflow + accepts slack_thread_ts.
  2. jitsucom/jitsu-cloud-infra#<PR> — deploy.yaml threads notifications and propagates thread_ts into the services.yaml dispatch.

Test plan

• Manually trigger Slack notify (reusable) via the Actions UI, verify it posts to #dev and outputs a ts.
• Re-trigger with thread_ts set to that ts, verify the reply lands in the thread.
• Re-trigger with update_ts set, verify the original message updates in place.
• Test the legacy webhook path by calling the composite action with only slack_webhook_url set.

[jitsu-code-review]

Validated the range a8928dc..200b7b1, including the new bot-token Slack path, webhook fallback, and reusable-workflow outputs wiring. I don’t see new actionable bugs, security issues, or correctness regressions in this change set.