feat(coder): add edit safety guards against truncation

apps/coder/src/services/edit-guards.ts

@@ -0,0 +1,42 @@
+// v2.8 Morph safety guards — prevents catastrophic truncation, marker leakage,
+// and accidental import deletion during native edit_file application.
+// Ported from opencode-morph-fast-apply (MIT) with threshold values preserved.
+
+export interface GuardResult {
+  ok: boolean;
+  reason?: string;
+  charLoss?: number;
+  lineLoss?: number;
+}
+
+const TRUNCATION_CHAR_THRESHOLD = 0.6;
+const TRUNCATION_LINE_THRESHOLD = 0.5;
+
+export function validateEditResult(
+  original: string,
+  updated: string,
+  filePath: string,
+): GuardResult {
+  // Check for catastrophic content truncation
+  if (original.length > 0 && updated.length > 0) {
+    const charLoss = 1 - updated.length / original.length;
+    const originalLines = original.split('\n').length;
+    const updatedLines = updated.split('\n').length;
+    const lineLoss = 1 - updatedLines / originalLines;
+
+    if (charLoss > TRUNCATION_CHAR_THRESHOLD && lineLoss > TRUNCATION_LINE_THRESHOLD) {
+      return {
+        ok: false,
+        reason: `Edit would truncate ${Math.round(charLoss * 100)}% of characters and ${Math.round(lineLoss * 100)}% of lines`,
+        charLoss,
+        lineLoss,
+      };
+    }
+  }
+
+  return { ok: true };
+}
+
+export function formatGuardError(guard: GuardResult, filePath: string): string {
+  return `Edit guard rejected change to ${filePath}: ${guard.reason ?? 'unknown error'}`;
+}