docs: HARNESS_PRIMER + INVARIANTS marker for route-versioning enforcement (#130, #132) (#76)

Author: constk

docs/HARNESS.md

@@ -32,6 +32,10 @@ The harness is independent of the project's domain. The included `src/api/echo`
 
 ## Reading order
 
+For someone who hasn't worked in a harness like this before, start with [`docs/HARNESS_PRIMER.md`](HARNESS_PRIMER.md) — it's the same surface as this doc but written for non-engineers.
+
+For an engineer setting up the template:
+
 1. **`docs/INVARIANTS.md`** — the load-bearing rules. Every PR is checked against them.
 2. **`docs/BOUNDARIES.md`** — module layering and the import-linter contracts.
 3. **`docs/DEVELOPMENT.md`** — local setup, the `justfile`, the CI pipeline.