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.

search/README.md

@@ -8,7 +8,7 @@ aggregation queries.
 
 Tantivy is a Rust library with no maintained Go bindings — using it from
 `ingest` (Go) would mean cgo-bridging to a compiled Rust cdylib, exactly
-the fragile FFI complexity CLAUDE.md's "prefer boring, well-understood
+the fragile FFI complexity PROJECT-SPEC.md's "prefer boring, well-understood
 dependencies... operators need to trust it" principle steers away from.
 It would also couple ClickHouse-write latency to Tantivy-write latency in
 the same request path. See `/docs/architecture.md` for the fuller