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/localauth/store.go

@@ -2,7 +2,7 @@
 // login: a real login page and session-based auth covering both /api
 // and /alerting, plus a simple admin-managed user list, for deployments
 // reachable over the internet that can no longer rely on Phase 0-3's
-// "no auth yet" default (see /docs/architecture.md and CLAUDE.md's
+// "no auth yet" default (see /docs/architecture.md and PROJECT-SPEC.md's
 // Phase 4 section for the enterprise/ SSO alternative this is not --
 // this package has no tenant/RBAC-service concept, just "is this a
 // valid logged-in user").