ci: align Mergify queue checks with main

[gildesmarais]

Summary

• switch the Mergify base branch condition from master to main
• require both docker-test matrix checks in the dependabot merge queue

Verification

• docker compose -f .devcontainer/docker-compose.yml up -d
• docker exec devcontainer-app-1 bash -lc 'cd /workspace && make setup && make ready'

Notes

• this slice changes only .mergify.yml and was rebuilt from main after feat: link included feeds to the instance feed directory #901 merged
• the Dev Container mount for linked worktrees does not preserve in-container git metadata, so verification was executed from /workspace without relying on git commands inside the container