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.

terraform/main.go

@@ -1,5 +1,5 @@
 // Command terraform-provider-cairnobs is Cairn OBS's Terraform provider --
-// CLAUDE.md names it a first-class deliverable alongside cairnobsctl
+// PROJECT-SPEC.md names it a first-class deliverable alongside cairnobsctl
 // ("CLI and Terraform provider are first-class, not afterthoughts"),
 // but this is the first phase to actually build any of it.
 //