feat: implement T01-T04 — Go module, canonical model, LDAP validator, error taxonomy

- T01: Go module (keycape), full directory skeleton, Makefile, CI workflow
- T02: spec/canonical-model.yaml with 6 entities + Go domain types
- T03: spec/ldap-schema.yaml + validator binary with structural/semantic rules
- T04: Error taxonomy — 4 stable error types, JSON format, HTTP helpers

28 tests pass, go vet clean, go build clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/cmd/lldap-export/main.go

@@ -0,0 +1,13 @@
+// lldap-export exports the LLDAP directory as a canonical YAML snapshot
+// for use with the validator and migration tools.
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	fmt.Fprintln(os.Stderr, "lldap-export: not yet implemented (T06+)")
+	os.Exit(1)
+}