feat(2c): end the core/captcha carve-out — zero block/core in standalone plugins

Captcha is now the host-stamped X-Bn-Verified-Captcha trusted header
(pluginsdk v0.2.2), so no plugin has a legitimate core import left:
isForbiddenPluginImport flags ALL first-party prefixes unconditionally,
and a block/core require in a plugin go.mod fails as
no-block-core-require (always vestigial — imports are already
forbidden). Fleet-verified: no plugin imports or requires core.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

plugin_imports.go

@@ -18,34 +18,19 @@ const (
 	firstPartyImportPrefix   = "git.dev.alexdunmow.com/block/"
 )
 
-// allowedCorePluginImports are the block/core packages that deliberately
-// stayed on core after the pluginsdk extraction (2026-07-07) and remain
-// plugin-usable; everything else first-party is host-only.
-var allowedCorePluginImports = []string{
-	blockCoreImportPrefix + "/captcha",
-	blockCoreImportPrefix + "/backup",
-}
-
 func hasImportPrefix(importPath, prefix string) bool {
 	return importPath == prefix || strings.HasPrefix(importPath, prefix+"/")
 }
 
 // isForbiddenPluginImport reports whether a standalone plugin may not import
-// the package: all BlockNinja first-party Go code (cms, orchestrator, core)
-// is off-limits except the plugin SDK and the explicitly kept core packages.
+// the package: ALL BlockNinja first-party Go code (cms, orchestrator, core) is
+// off-limits — block/pluginsdk is the one plugin-facing module. (The former
+// core/captcha carve-out ended when captcha moved to the host-stamped
+// X-Bn-Verified-Captcha trusted header, pluginsdk v0.2.2.)
 func isForbiddenPluginImport(importPath string) bool {
-	if hasImportPrefix(importPath, blockNinjaImportPrefix) || hasImportPrefix(importPath, orchestratorImportPrefix) {
-		return true
-	}
-	if hasImportPrefix(importPath, blockCoreImportPrefix) {
-		for _, allowed := range allowedCorePluginImports {
-			if hasImportPrefix(importPath, allowed) {
-				return false
-			}
-		}
-		return true
-	}
-	return false
+	return hasImportPrefix(importPath, blockNinjaImportPrefix) ||
+		hasImportPrefix(importPath, orchestratorImportPrefix) ||
+		hasImportPrefix(importPath, blockCoreImportPrefix)
 }
 
 type pluginImportViolation struct {

plugin_imports_test.go

@@ -51,22 +51,20 @@ func Example() {
 	}
 }
 
-func TestCheckStandalonePluginImportsAllowsSDKAndKeptCoreImports(t *testing.T) {
+func TestCheckStandalonePluginImportsAllowsSDKImports(t *testing.T) {
 	root := t.TempDir()
 	writeTestFile(t, filepath.Join(root, "plugin.mod"), "[plugin]\nname = \"example\"\nversion = \"1.0.0\"\n", 0644)
 	writeTestFile(t, filepath.Join(root, "main.go"), `package example
 
 import (
-	"git.dev.alexdunmow.com/block/core/backup"
-	"git.dev.alexdunmow.com/block/core/captcha"
+	"git.dev.alexdunmow.com/block/pluginsdk/auth"
 	"git.dev.alexdunmow.com/block/pluginsdk/plugin"
 	"git.dev.alexdunmow.com/block/pluginsdk/render"
 )
 
 func Example() {
 	_ = plugin.PluginRegistration{}
-	_ = captcha.Config{}
-	_ = backup.Config{}
+	_ = auth.CaptchaVerified
 	_ = render.BlockNoteToHTML
 }
 `, 0644)
@@ -77,6 +75,30 @@ func Example() {
 	}
 }
 
+// Captcha moved to the host-stamped X-Bn-Verified-Captcha trusted header
+// (pluginsdk/auth), so the former core/captcha carve-out is gone: NO block/core
+// package is plugin-importable anymore.
+func TestCheckStandalonePluginImportsFlagsFormerlyKeptCoreImports(t *testing.T) {
+	root := t.TempDir()
+	writeTestFile(t, filepath.Join(root, "plugin.mod"), "[plugin]\nname = \"example\"\nversion = \"1.0.0\"\n", 0644)
+	writeTestFile(t, filepath.Join(root, "main.go"), `package example
+
+import "git.dev.alexdunmow.com/block/core/captcha"
+
+func Example() {
+	_ = captcha.New
+}
+`, 0644)
+
+	violations := checkStandalonePluginImports(root)
+	if len(violations) != 1 {
+		t.Fatalf("checkStandalonePluginImports() returned %d violations, want 1: %#v", len(violations), violations)
+	}
+	if violations[0].importPath != "git.dev.alexdunmow.com/block/core/captcha" {
+		t.Fatalf("importPath = %q, want forbidden core/captcha import", violations[0].importPath)
+	}
+}
+
 func TestCheckStandalonePluginImportsFlagsOrchestratorImports(t *testing.T) {
 	root := t.TempDir()
 	writeTestFile(t, filepath.Join(root, "plugin.mod"), "[plugin]\nname = \"example\"\nversion = \"1.0.0\"\n", 0644)

plugin_sdk_versions.go

@@ -169,13 +169,25 @@ func checkStandalonePluginGoMod(root, requiredSDKVersion string) []pluginGoModVi
 	violations = append(violations, checkGoModForAnyReplaceDirectives(goModPath)...)
 
 	// The standalone-plugin SDK is block/pluginsdk (2026-07-07 extraction).
-	// A block/core require may coexist for the packages that stayed core
-	// (captcha, backup) — allowed, not required.
+	// block/core is host-only: no plugin package imports survive (check 2c
+	// imports), so a core require is always vestigial and fails the gate.
 	currentSDKVersion := ""
 	for _, req := range parsed.Require {
 		if req.Mod.Path == pluginSDKImportPrefix {
 			currentSDKVersion = req.Mod.Version
-			break
+			continue
+		}
+		if req.Mod.Path == blockCoreImportPrefix {
+			line := 0
+			if req.Syntax != nil {
+				line = req.Syntax.Start.Line
+			}
+			violations = append(violations, pluginGoModViolation{
+				file:   "go.mod",
+				line:   line,
+				rule:   "no-block-core-require",
+				detail: fmt.Sprintf("requires %s — standalone plugins build against %s only", blockCoreImportPrefix, pluginSDKImportPrefix),
+			})
 		}
 	}
 	if currentSDKVersion == "" {

plugin_sdk_versions_test.go

@@ -116,7 +116,7 @@ func TestCheckStandalonePluginGoModFlagsMissingPluginSDKRequire(t *testing.T) {
 
 go 1.26.2
 
-require git.dev.alexdunmow.com/block/core v0.2.1
+require github.com/google/uuid v1.6.0
 `, 0644)
 
 	violations := checkStandalonePluginGoMod(root, "v0.2.1")
@@ -128,14 +128,30 @@ require git.dev.alexdunmow.com/block/core v0.2.1
 	}
 }
 
-// A coexisting block/core require is allowed alongside the required pluginsdk
-// require (e.g. calcomblock keeps core for captcha).
 func TestCheckStandalonePluginGoModAllowsMatchingSDKVersionWithoutReplace(t *testing.T) {
 	root := t.TempDir()
 	writeTestFile(t, filepath.Join(root, "go.mod"), `module example.com/plugin
 
 go 1.26.2
 
+require git.dev.alexdunmow.com/block/pluginsdk v0.2.1
+`, 0644)
+
+	violations := checkStandalonePluginGoMod(root, "v0.2.1")
+	if len(violations) != 0 {
+		t.Fatalf("checkStandalonePluginGoMod() returned %d violations, want 0: %#v", len(violations), violations)
+	}
+}
+
+// The former captcha/backup coexistence is over (captcha = host-stamped
+// trusted header since pluginsdk v0.2.2): a block/core require in a
+// standalone plugin go.mod is always vestigial and now fails the gate.
+func TestCheckStandalonePluginGoModFlagsCoreRequire(t *testing.T) {
+	root := t.TempDir()
+	writeTestFile(t, filepath.Join(root, "go.mod"), `module example.com/plugin
+
+go 1.26.2
+
 require (
 	git.dev.alexdunmow.com/block/core v0.5.0
 	git.dev.alexdunmow.com/block/pluginsdk v0.2.1
@@ -143,8 +159,11 @@ require (
 `, 0644)
 
 	violations := checkStandalonePluginGoMod(root, "v0.2.1")
-	if len(violations) != 0 {
-		t.Fatalf("checkStandalonePluginGoMod() returned %d violations, want 0: %#v", len(violations), violations)
+	if len(violations) != 1 {
+		t.Fatalf("checkStandalonePluginGoMod() returned %d violations, want 1: %#v", len(violations), violations)
+	}
+	if violations[0].rule != "no-block-core-require" {
+		t.Fatalf("rule = %q, want no-block-core-require", violations[0].rule)
 	}
 }