feat(ci): release canary branch automation

[ludamad]

Summary

• Adds the "Alpha Team Canary" workflow that triggers on push to next when **/bootstrap.sh or ci3/** files change
• The workflow:
  • Ensures the canary branch and PR exist (creates them if needed)
  • Checks if CI is already running before updating to avoid duplicate runs
  • Merges next into the canary PR branch (ad/chore/ci-release-pr-canary)
  • Notifies #alpha-team Slack channel on workflow failure
• Adds ci3/alpha_team_canary_slack_notify script for Slack notifications (matches merge train pattern)
• Adds CI failure notification call to ci3/bootstrap_ec2 alongside merge train notifications

This works together with the canary PR #19936 which is a long-running PR with the ci-release-pr label for testing CI release processes.

[AztecBot]

Flakey Tests

🤖 says: This CI run detected 2 tests that failed, but were tolerated due to a .test_patterns.yml entry.

\033FLAKED\033 (8;;http://ci.aztec-labs.com/5f1d66d041eb5e22�5f1d66d041eb5e228;;�):  yarn-project/end-to-end/scripts/run_test.sh simple src/e2e_p2p/rediscovery.test.ts (204s) (code: 1) group:e2e-p2p-epoch-flakes (\033ludamad\033: feat(ci): release canary branch automation (#19937))
\033FLAKED\033 (8;;http://ci.aztec-labs.com/7ff300a3274c4c62�7ff300a3274c4c628;;�):  yarn-project/end-to-end/scripts/run_test.sh simple src/e2e_p2p/valid_epoch_pruned_slash.test.ts (394s) (code: 1) group:e2e-p2p-epoch-flakes (\033ludamad\033: feat(ci): release canary branch automation (#19937))