Add copilot-setup-steps.yml so the Copilot agent can install deps and run checks #74

The Copilot cloud agent builds on a fresh GitHub Actions VM and currently has to guess how to install this repo and run its checks. Add .github/workflows/copilot-setup-steps.yml so the environment is ready before the agent starts.

Reference for what CI already does: .github/workflows/checks.yml.

Acceptance criteria:

• .github/workflows/copilot-setup-steps.yml defines a single job named exactly copilot-setup-steps (the name GitHub requires), triggered by workflow_dispatch plus push/pull_request on changes to that file only, so it can be validated in CI.
• The job checks out the repo and installs the toolchain and dependencies at the versions CI uses (same runtime versions and lockfile install command), with dependency caching.
• It provisions any service the test suite needs on the VM (for example a postgres:16 service container) with the same env vars CI uses. No real secrets, no production or staging URLs.
• Permissions are minimal (contents: read).
• It ends by running the repo's fast check command once, so a broken setup fails visibly.
• No other workflow or application code changes.
• Test databases need migrate_db(); getDb() alone does not migrate.