fix: reduce session.messages() calls with event-based caching to prevent memory leaks

- Replace session.messages() fetch in context-window-monitor with message.updated event cache
- Replace session.messages() fetch in preemptive-compaction with message.updated event cache
- Add per-session transcript cache (5min TTL) to avoid full rebuild per tool call
- Remove session.messages() from background-agent polling (use event-based progress)
- Add TTL pruning to todo-continuation-enforcer session state Map
- Add setInterval.unref() to tool-input-cache cleanup timer

Fixes #1222

src/hooks/claude-code-hooks/transcript.test.ts

@@ -0,0 +1,102 @@
+import { describe, it, expect, mock, beforeEach, afterEach } from "bun:test"
+import { existsSync, unlinkSync, readFileSync } from "fs"
+import {
+  buildTranscriptFromSession,
+  deleteTempTranscript,
+  clearTranscriptCache,
+} from "./transcript"
+
+function createMockClient(messages: unknown[] = []) {
+  return {
+    session: {
+      messages: mock(() =>
+        Promise.resolve({
+          data: messages,
+        })
+      ),
+    },
+  }
+}
+
+describe("transcript caching", () => {
+  afterEach(() => {
+    clearTranscriptCache()
+  })
+
+  // #given same session called twice
+  // #when buildTranscriptFromSession is invoked
+  // #then session.messages() should be called only once (cached)
+  it("should cache transcript and not re-fetch for same session", async () => {
+    const client = createMockClient([
+      {
+        info: { role: "assistant" },
+        parts: [
+          {
+            type: "tool",
+            tool: "bash",
+            state: { status: "completed", input: { command: "ls" } },
+          },
+        ],
+      },
+    ])
+
+    const path1 = await buildTranscriptFromSession(
+      client,
+      "ses_cache1",
+      "/tmp",
+      "bash",
+      { command: "echo hi" }
+    )
+
+    const path2 = await buildTranscriptFromSession(
+      client,
+      "ses_cache1",
+      "/tmp",
+      "read",
+      { path: "/tmp/file" }
+    )
+
+    // session.messages() called only once
+    expect(client.session.messages).toHaveBeenCalledTimes(1)
+
+    // Both return valid paths
+    expect(path1).not.toBeNull()
+    expect(path2).not.toBeNull()
+
+    // Second call should append the new tool entry
+    if (path2) {
+      const content = readFileSync(path2, "utf-8")
+      expect(content).toContain("Read")
+    }
+
+    deleteTempTranscript(path1)
+    deleteTempTranscript(path2)
+  })
+
+  // #given different sessions
+  // #when buildTranscriptFromSession called for each
+  // #then session.messages() should be called for each
+  it("should not share cache between different sessions", async () => {
+    const client = createMockClient([])
+
+    await buildTranscriptFromSession(client, "ses_a", "/tmp", "bash", {})
+    await buildTranscriptFromSession(client, "ses_b", "/tmp", "bash", {})
+
+    expect(client.session.messages).toHaveBeenCalledTimes(2)
+
+    clearTranscriptCache()
+  })
+
+  // #given clearTranscriptCache is called
+  // #when buildTranscriptFromSession called again
+  // #then should re-fetch
+  it("should re-fetch after cache is cleared", async () => {
+    const client = createMockClient([])
+
+    await buildTranscriptFromSession(client, "ses_clear", "/tmp", "bash", {})
+    clearTranscriptCache()
+    await buildTranscriptFromSession(client, "ses_clear", "/tmp", "bash", {})
+
+    expect(client.session.messages).toHaveBeenCalledTimes(2)
+  })
+})