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.

docs/phase-4-isolation-design.md

@@ -37,7 +37,7 @@ ships entirely in `enterprise/`, not AGPL core. Core (`/api`,
 `/alerting`, `/web`) stays genuinely single-tenant: no multi-tenant
 mechanism present at all, not merely a missing management UI on top of
 otherwise-functional isolation. This was an explicit choice put to the
-project owner rather than assumed, because CLAUDE.md's licensing
+project owner rather than assumed, because PROJECT-SPEC.md's licensing
 boundary text names multi-tenancy as enterprise-gated, and a
 "mechanism in core, feature in enterprise" split would have let a
 sufficiently motivated self-hosting AGPL user wire up real isolation