test(guard): add merge-conflict guard to prevent unresolved git conflicts in source files

Unresolved git merge conflict markers (<<<<<<<, =======, >>>>>>>) in
TypeScript source files break parsing and can cause the plugin to fail
at runtime or tests to hang with cryptic errors. This guard scans all
.ts/.tsx/.json files under src/ and fails the test suite if any
conflict markers are found.

Closes #debugging-hang-investigation

src/hooks/shared/merge-conflict-guard.test.ts

@@ -0,0 +1,39 @@
+import { describe, expect, test } from "bun:test"
+import { readdirSync, readFileSync } from "fs"
+import { join } from "path"
+
+function* walk(dir: string): Generator<string> {
+  for (const entry of readdirSync(dir, { withFileTypes: true })) {
+    const path = join(dir, entry.name)
+    if (entry.isDirectory()) {
+      if (entry.name === "node_modules" || entry.name === ".git" || entry.name === "dist") {
+        continue
+      }
+      yield* walk(path)
+    } else if (entry.isFile() && (entry.name.endsWith(".ts") || entry.name.endsWith(".tsx") || entry.name.endsWith(".json"))) {
+      yield path
+    }
+  }
+}
+
+function hasConflictMarkers(content: string): boolean {
+  const lines = content.split("\n")
+  return lines.some((line) =>
+    line.startsWith("<<<<<<< ") ||
+    line === "=======" ||
+    line.startsWith(">>>>>>> ")
+  )
+}
+
+describe("#given source files in src/", () => {
+  test("#then no file contains unresolved git merge conflict markers", () => {
+    const conflicts: string[] = []
+    for (const path of walk(join(import.meta.dir, "../../../src"))) {
+      const content = readFileSync(path, "utf-8")
+      if (hasConflictMarkers(content)) {
+        conflicts.push(path)
+      }
+    }
+    expect(conflicts).toEqual([])
+  })
+})