Publish schema target and compatibility floor

Why: a rollback can only be safe when the candidate says which schema it
will migrate to and whether older binaries can still read that schema.
A new migration that forgets to register that fact must fail the test.
Evidence: go test ./internal/store/ ./pkg/identity/ -run 'TestMigrationCompat|TestCapabilities' passed.
Impact: version --json and identity capabilities include schema.target and
schema.compatFloor. Every migration version is listed in
migrationBreaksOldReaders. Current versions do not break old readers, so
the floor stays 0.
Refs: recovery plan r3.1 D2

Co-authored-by: Cursor <cursoragent@cursor.com>

pkg/identity/identity.go

@@ -14,6 +14,7 @@ import (
 	"strings"
 
 	"agent-system-backend/internal/app"
+	"agent-system-backend/internal/store"
 )
 
 // Usage 无参/未知子命令时打印到 stderr 的用法行。探测契约：同时含 "identity"
@@ -65,6 +66,7 @@ func NewDocument(product, version string) Document {
 		Version:  version,
 		Identity: IdentityCaps{Ops: []string{"ensure", "verify"}, Open: "existing-no-migrate", DSN: "env"},
 		CLI:      CLICaps{UnknownSubcommand: "reject"},
+		Schema:   &SchemaCaps{Target: store.SchemaTarget(), CompatFloor: store.SchemaCompatFloor()},
 	}
 }
 

pkg/identity/identity_test.go

@@ -61,6 +61,9 @@ func TestCapabilities(t *testing.T) {
 	if doc.Protocol != 1 || doc.Identity.Open != "existing-no-migrate" || doc.CLI.UnknownSubcommand != "reject" {
 		t.Fatalf("能力文档异常：%+v", doc)
 	}
+	if doc.Schema == nil || doc.Schema.Target != store.SchemaTarget() || doc.Schema.CompatFloor != store.SchemaCompatFloor() {
+		t.Fatalf("schema 能力异常：%+v", doc.Schema)
+	}
 }
 
 func migratedData(t *testing.T) (string, int) {