feat(coder): add edit safety guards against truncation

apps/coder/src/services/pending_changes.ts

@@ -4,6 +4,7 @@ import { randomBytes } from 'node:crypto';
 import type { Sql } from '../db.js';
 import { resolveWritePath } from './write_guard.js';
 import { locateMatch } from './fuzzy-match.js';
+import { validateEditResult, formatGuardError } from './edit-guards.js';
 
 /**
  * Write a file atomically: stage to a sibling temp file, then rename over the
@@ -285,6 +286,10 @@ export async function applyOne(
             );
           }
           if (plan.kind === 'apply') {
+            const guard = validateEditResult(toLf(raw), plan.updated, change.file_path);
+            if (!guard.ok) {
+              throw new Error(formatGuardError(guard, change.file_path));
+            }
             const out = eol === '\r\n' ? plan.updated.replaceAll('\n', '\r\n') : plan.updated;
             await writeFileAtomic(change.file_path, out);
           } else {