feat(atlas): enhance session-last-agent with timestamp-based ordering

- Sort messages by creation timestamp for accurate last agent detection
- Add fallback to filename sorting for deterministic ordering
- Add JSON backend test coverage
- Update SQLite backend tests for timestamp-aware sorting

🤖 Generated with assistance of OhMyOpenCode

src/hooks/atlas/session-last-agent.json.test.ts

@@ -0,0 +1,67 @@
+declare const require: (name: string) => any
+const { afterEach, describe, expect, mock, test, afterAll } = require("bun:test")
+import { mkdtempSync, mkdirSync, rmSync, writeFileSync } from "node:fs"
+import { join } from "node:path"
+import { tmpdir } from "node:os"
+
+const testDirs: string[] = []
+const TEST_STORAGE_ROOT = join(tmpdir(), `atlas-session-last-agent-${Date.now()}`)
+const TEST_MESSAGE_STORAGE = join(TEST_STORAGE_ROOT, "message")
+
+mock.module("../../shared/opencode-storage-detection", () => ({
+  isSqliteBackend: () => false,
+}))
+
+mock.module("../../shared/opencode-message-dir", () => ({
+  getMessageDir: (sessionID: string) => {
+    const directPath = join(TEST_MESSAGE_STORAGE, sessionID)
+    return require("node:fs").existsSync(directPath) ? directPath : null
+  },
+}))
+
+afterAll(() => { mock.restore() })
+
+afterEach(() => {
+  while (testDirs.length > 0) {
+    const directory = testDirs.pop()
+    if (directory) {
+      rmSync(directory, { recursive: true, force: true })
+    }
+  }
+})
+
+function createTempMessageDir(sessionID: string): string {
+  const directory = mkdtempSync(join(tmpdir(), "atlas-session-last-agent-json-"))
+  testDirs.push(directory)
+  const messageDir = join(TEST_MESSAGE_STORAGE, sessionID)
+  mkdirSync(messageDir, { recursive: true })
+  return messageDir
+}
+
+describe("getLastAgentFromSession JSON backend", () => {
+  test("returns the newest non-compaction agent by message timestamp rather than filename order", async () => {
+    // given
+    const sessionID = "ses_json_last_agent"
+    const messageDir = createTempMessageDir(sessionID)
+    writeFileSync(join(messageDir, "msg_ffff0000_000001.json"), JSON.stringify({
+      agent: "compaction",
+      time: { created: 200 },
+    }), "utf-8")
+    writeFileSync(join(messageDir, "msg_00000000_000999.json"), JSON.stringify({
+      agent: "atlas",
+      time: { created: 100 },
+    }), "utf-8")
+    writeFileSync(join(messageDir, "msg_11111111_000002.json"), JSON.stringify({
+      agent: "sisyphus-junior",
+      time: { created: 50 },
+    }), "utf-8")
+
+    const { getLastAgentFromSession } = await import("./session-last-agent")
+
+    // when
+    const result = await getLastAgentFromSession(sessionID)
+
+    // then
+    expect(result).toBe("atlas")
+  })
+})