Rename the project spec and update every reference to it

The charter file carried a tool-specific name while being the repository's own
document: mission, non-negotiable constraints, the pinned stack, repo
conventions and phase status, cited as authority by thirty files across the
agent, api, deploy, docs, search and terraform trees.

PROJECT-SPEC.md says what it is. All 42 references are updated in the same
commit, including the relative link in docs/status.md, so nothing points at a
filename that no longer exists.

enterprise/README.md

@@ -206,7 +206,7 @@ below) and Tantivy (`/search/README.md`'s "Per-tenant indices" section
 routing (does traffic actually reach `enterprise-api` instead of
 `api`), now a single-flag choice in both `deploy/helm/cairnobs` and
 `docker-compose.yml` (`enterprise.enabled` / `COMPOSE_PROFILES`), see
-CLAUDE.md.
+PROJECT-SPEC.md.
 
 ## Ingest tenant identity