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/security/threat-model.md

@@ -495,7 +495,7 @@ handler to call them from).
 
 ## Known residual risks (explicitly out of scope, not silently assumed away)
 
-Per `/CLAUDE.md`'s Phase 4 non-goals, restated here in threat-model
+Per `/PROJECT-SPEC.md`'s Phase 4 non-goals, restated here in threat-model
 terms:
 
 - **A privileged ClickHouse/Postgres administrator is not defended