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>

internal/store/migrate_compat.go

@@ -0,0 +1,49 @@
+package store
+
+// migrationBreaksOldReaders 登记每个迁移版本落地后，schema.target 更低的旧制品
+// 还能不能读这个库。新增迁移漏登记时 TestMigrationCompatRegistered 失败。
+// true 会抬高 SchemaCompatFloor：候选若要把库迁过该版本，而旧制品的 target
+// 低于该下限，切换必须在停旧服务前拒绝，除非走已确认的 forward-only 迁移。
+var migrationBreaksOldReaders = map[int]bool{
+	1:  false,
+	2:  false,
+	3:  false,
+	4:  false,
+	5:  false,
+	6:  false,
+	7:  false,
+	8:  false,
+	9:  false,
+	10: false,
+	11: false,
+	12: false,
+	13: false,
+	14: false,
+	15: false,
+	16: false,
+	17: false,
+	18: false,
+	19: false,
+	20: false,
+	21: false,
+	22: false,
+	23: false,
+	24: false,
+	25: false,
+	26: false,
+	27: false,
+}
+
+// SchemaTarget 是本二进制会迁移到的 schema 版本。
+func SchemaTarget() int { return latestMigrationVersion() }
+
+// SchemaCompatFloor 是旧制品仍能读取的最高破坏性版本。0 表示当前全部迁移都不破坏旧读。
+func SchemaCompatFloor() int {
+	floor := 0
+	for version, breaks := range migrationBreaksOldReaders {
+		if breaks && version > floor {
+			floor = version
+		}
+	}
+	return floor
+}

internal/store/migrate_compat_test.go

@@ -0,0 +1,20 @@
+package store
+
+import "testing"
+
+func TestMigrationCompatRegistered(t *testing.T) {
+	if len(migrationBreaksOldReaders) != len(migrations) {
+		t.Fatalf("兼容登记 %d 条，迁移 %d 条", len(migrationBreaksOldReaders), len(migrations))
+	}
+	for _, m := range migrations {
+		if _, ok := migrationBreaksOldReaders[m.version]; !ok {
+			t.Fatalf("迁移 v%d %s 未登记是否破坏旧制品读取", m.version, m.name)
+		}
+	}
+	if SchemaTarget() != latestMigrationVersion() {
+		t.Fatalf("target=%d latest=%d", SchemaTarget(), latestMigrationVersion())
+	}
+	if SchemaCompatFloor() < 0 || SchemaCompatFloor() > SchemaTarget() {
+		t.Fatalf("compatFloor=%d target=%d", SchemaCompatFloor(), SchemaTarget())
+	}
+}

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) {