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-2-runbook.md

@@ -97,7 +97,7 @@ not a projection):
 | `SELECT count() FROM logs WHERE service='web'` (raw SQL) | Pure ClickHouse | 17.5ms |
 
 All four "well under a second" — the Phase 2 exit criteria in
-`/CLAUDE.md`. The combined text+aggregation case (the one everyone should
+`/PROJECT-SPEC.md`. The combined text+aggregation case (the one everyone should
 be nervous about, since it's a two-backend query) came in at 46ms, not
 meaningfully slower than the pure-ClickHouse case — the Tantivy prefilter
 step is fast, and 5,000 UUIDs (see below) is a small `IN` clause by