fix(examples): stop tracking generated build output

[devpie]

Description

examples/opa-policy-reactive-agent/build/config.json is a build artifact, not source. 060-policies-config.tf writes it through a local_file resource on every terraform apply, so the checked-in copy was generated output that got committed by mistake in 71d9bfc.

Two consequences:

• The committed copy carried the STACKIT project ID and the policy-engine service account address of the environment it was generated in.
• Any terraform apply rewrote the file and left it as a spurious diff in the contributor's working tree.

This PR removes the file and adds build/ to .gitignore. The pattern has no leading slash, so it covers any build/ directory anywhere in the repo, not just this one.

Notes for reviewers

• Nothing reads the file. The policy engine's config is rendered on the VM by cloud-init/policy-engine.yaml.tftpl through templatefile() (see 050-policy-engine-host.tf), and scripts/policy_runner.py reads /etc/stackit-policy-engine/config.json there. Removing the local artifact does not change the example; the next apply regenerates it.
• Removing the file does not remove it from history. The values remain readable at commit 71d9bfc and have been public since that commit was mirrored to GitHub. Whether history needs a rewrite, or the service account needs rotating, is a decision for the maintainers and the account owner — this PR deliberately does not touch it.
• The deleted file had mode 100755, an executable JSON file.
• Verified locally: check_readme_tags.py, check_todos.py, generate_agents_md.py --check and the pre-commit hygiene hooks all pass. The ignore pattern was tested against build/ directories at two different depths, including one outside examples/.

Checklist

• The CI pipeline passed successfully.

🤖 Generated with Claude Code

[h3adex]

Thx!