fix: prevent duplicate AGENTS.md injection when reading instruction files (#11581)

Co-authored-by: Aiden Cline <aidenpcline@gmail.com>

packages/opencode/test/session/instruction.test.ts

@@ -47,4 +47,24 @@ describe("InstructionPrompt.resolve", () => {
       },
     })
   })
+
+  test("doesn't reload AGENTS.md when reading it directly", async () => {
+    await using tmp = await tmpdir({
+      init: async (dir) => {
+        await Bun.write(path.join(dir, "subdir", "AGENTS.md"), "# Subdir Instructions")
+        await Bun.write(path.join(dir, "subdir", "nested", "file.ts"), "const x = 1")
+      },
+    })
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const filepath = path.join(tmp.path, "subdir", "AGENTS.md")
+        const system = await InstructionPrompt.systemPaths()
+        expect(system.has(filepath)).toBe(false)
+
+        const results = await InstructionPrompt.resolve([], filepath, "test-message-2")
+        expect(results).toEqual([])
+      },
+    })
+  })
 })