Define what the OpenAPI info.version means and add a changelog #511

Follow-up from the change policy (docs/reference/change-policy.md, PR #506).

Both docs/api/openapi.yaml and docs/api/harness-openapi.yaml say version: 1.0.0 with no stated meaning.

Decision: it is the version of the /v1 contract. Bump the minor for additive changes; bump the major only together with a /v2. Add a short changelog to each file (or a linked changelog) and mention the rule in the policy page.