fix(chat): keep a long turn's Edited N files card whole across the page cut (#86)

The reported turn (/vp-auto, ~120 messages) showed "Edited 1 file": the
chat loads the newest 50 messages, and the turn's other Write
(rbac_routes_test.go, +176) was in the part before the page.

- turnCarry (agent-edits.js): every message the route's sliding window
  drops goes through it; it keeps the file changes since the last prompt it
  dropped (a dropped call whose result is on the page, and dropped
  subagents, included).
- A Claude page that starts mid-turn answers them as `turn_edits`; the
  chat keeps the one from the page that holds its oldest message and adds
  it to that turn's card.

Files a turn changed through Bash (python/sed writing a file) are still not
listed: nothing in the transcript says which files a command wrote.

server/lib/agent-edits.js

@@ -8,7 +8,8 @@
  * subagent's own subagents are folded in, `MAX_DEPTH` levels down. The file is
  * read through `readLines` (a member's transcript only as that member's), and
  * the answer is cached by path + size + mtime, so a finished subagent is read
- * once however often its parent's chat is polled.
+ * once however often its parent's chat is polled. `turnCarry` does the same
+ * for the part of a turn the chat's page of messages cut off.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
@@ -21,6 +22,7 @@ const {
 } = require("./claude-home");
 
 const EDIT_TOOLS = new Set(["Write", "Edit", "MultiEdit", "NotebookEdit"]);
+const AGENT_TOOLS = new Set(["Agent", "Task"]);
 const MAX_DEPTH = 3;
 const CACHE_MAX = 500;
 // path → { key: "<size>:<mtime>", edits, agents }
@@ -129,4 +131,77 @@ async function attachAgentEdits(messages, sessionId, cwd) {
   }
 }
 
-module.exports = { agentEdits, attachAgentEdits, _cache: cache };
+/** A human prompt: the line that starts a turn (the chat's own rule). */
+const isPrompt = (m) =>
+  (m.sender ?? (m.type === "assistant" ? "assistant" : "user")) === "user" &&
+  (m.content ?? []).some((c) => c.type === "text");
+
+/**
+ * The edits of the turn a page of the chat starts in the middle of. The route
+ * keeps only the newest `limit` messages; every message it drops goes through
+ * `drop()`, which keeps the file changes since the last prompt it dropped.
+ * `result(window)` then answers the files that turn changed BEFORE the page's
+ * first message — `[{ path, added, removed, counted }]`, or null when the page
+ * starts on a prompt or nothing was changed — so the turn's "Edited N files"
+ * card is whole although its start was never sent (issue #86: a 120-message
+ * turn showed 1 of its files).
+ */
+function turnCarry() {
+  let pending = new Map(); // dropped Write/Edit call id → path
+  let pendingAgents = new Set(); // dropped Agent/Task call ids
+  let files = new Map();
+  let agents = [];
+  const add = (p, f) => {
+    const o = files.get(p) ?? { added: 0, removed: 0, counted: false };
+    o.added += f?.added ?? 0;
+    o.removed += f?.removed ?? 0;
+    o.counted ||= !!f && f.counted !== false;
+    files.set(p, o);
+  };
+  return {
+    drop(m) {
+      if (!m) return;
+      if (isPrompt(m)) {
+        pending = new Map();
+        pendingAgents = new Set();
+        files = new Map();
+        agents = [];
+        return;
+      }
+      for (const c of m.content ?? []) {
+        if (c.type === "tool_use" && c.id) {
+          if (AGENT_TOOLS.has(c.name)) pendingAgents.add(c.id);
+          if (!EDIT_TOOLS.has(c.name)) continue;
+          const p = c.input?.file_path ?? c.input?.notebook_path;
+          if (typeof p === "string" && p) pending.set(c.id, p);
+        } else if (c.type === "tool_result") {
+          if (c.agent?.id && !c.is_error) agents.push(c.agent.id);
+          pendingAgents.delete(c.id);
+          const p = pending.get(c.id);
+          if (!p) continue;
+          pending.delete(c.id);
+          if (!c.is_error) add(p, c.diff ?? null);
+        }
+      }
+    },
+    async result(window, sessionId, cwd) {
+      if (!window.length || isPrompt(window[0])) return null;
+      // A call dropped whose result is on the page: the page shows the result
+      // with no call to hang it on, so it counts here.
+      for (const m of window) {
+        for (const c of m.content ?? []) {
+          if (c.type !== "tool_result" || c.is_error) continue;
+          const p = pending.get(c.id);
+          if (p) add(p, c.diff ?? null);
+          if (pendingAgents.has(c.id)) for (const e of c.edits ?? []) add(e.path, e);
+        }
+      }
+      for (const id of agents) {
+        for (const e of await agentEdits(sessionId, cwd, id)) add(e.path, e);
+      }
+      return files.size ? [...files].map(([path, f]) => ({ path, ...f })) : null;
+    },
+  };
+}
+
+module.exports = { agentEdits, attachAgentEdits, turnCarry, _cache: cache };