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/README.md

@@ -1,6 +1,6 @@
 # terraform-provider-cairnobs
 
-Cairn OBS's Terraform provider -- `CLAUDE.md`'s "Repo conventions" section
+Cairn OBS's Terraform provider -- `PROJECT-SPEC.md`'s "Repo conventions" section
 names this a first-class deliverable alongside `cairnobsctl`
 ("CLI and Terraform provider are first-class, not afterthoughts"), but
 no phase before this one had actually built any of it. Four resources