infra/ contains no infrastructure — relocate the generated lab inventory #20

What: in a workshop that teaches infrastructure-as-code, a top-level infra/ directory strongly signals deployable HCL. It actually contains exactly one file: infra/lab-inventory.json, generated by scripts/lab-inventory.mjs --write as a machine mirror of docs/validation-matrix.md. Newcomers open it expecting Terraform and find derived docs data.

Fix: move the file to a location that says what it is (e.g. scripts/data/lab-inventory.json), update INVENTORY_PATH in scripts/lab-inventory.mjs, and grep for other references (rg lab-inventory across the repo, including CI and docs) so nothing points at the old path. Remove the empty infra/ dir.

Verify: node scripts/lab-inventory.mjs --check green; task verify green; no stale references.

Found by the 2026-08-30 project audit (finding ARCH-8).