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.

api/README.md

@@ -9,7 +9,7 @@ and `/docs/query-language-reference.md` for the user-facing syntax.
 
 ## Why plain REST, not gRPC + REST gateway
 
-CLAUDE.md pins the control plane to "Go, gRPC + REST gateway." This
+PROJECT-SPEC.md pins the control plane to "Go, gRPC + REST gateway." This
 service is plain `net/http` instead — a deliberate simplification, not a
 change to the pinned stack. Wiring up a `.proto` service,
 `google.api.http` annotations, and `protoc-gen-grpc-gateway` codegen for