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/routes/sessions.js

@@ -1177,6 +1177,9 @@ router.get("/:id/transcript", async (req, res) => {
             : (e, n) => parseMessage(e, n);
 
     const parseMessage = createClaudeLineParser(isSubagentFile);
+    const claudeFile = !isCodex && !isOmp && !isCursor && !isAgy;
+    // The edits of the turn the page starts inside of (agent-edits.js).
+    const carry = claudeFile ? require("../lib/agent-edits").turnCarry() : null;
 
     if (afterLine !== null) {
       // Incremental mode: skip lines until after afterLine, collect up to limit, then stop.
@@ -1232,7 +1235,7 @@ router.get("/:id/transcript", async (req, res) => {
         messages.push(message);
         // Sliding window: only keep the last `limit` messages
         if (messages.length > limit) {
-          messages.shift();
+          carry?.drop(messages.shift());
         }
       }
       if (total > limit) hasMore = true;
@@ -1283,7 +1286,7 @@ router.get("/:id/transcript", async (req, res) => {
         messages.push(message);
         // Sliding window: only keep the last `limit` messages in memory
         if (messages.length > limit) {
-          messages.shift();
+          carry?.drop(messages.shift());
         }
       }
       // If we shifted any messages out, there are more
@@ -1292,8 +1295,10 @@ router.get("/:id/transcript", async (req, res) => {
 
     // A Claude subagent's own Write/Edit calls are in ITS transcript: put the
     // files on its Agent result, so the turn's "Edited N files" counts them.
-    if (!isCodex && !isOmp && !isCursor && !isAgy) {
+    let turnEdits = null;
+    if (claudeFile) {
       await require("../lib/agent-edits").attachAgentEdits(messages, req.params.id, session.cwd);
+      turnEdits = await carry.result(messages, req.params.id, session.cwd);
     }
 
     const lastLine = messages.length > 0 ? messages[messages.length - 1].line : 0;
@@ -1310,6 +1315,7 @@ router.get("/:id/transcript", async (req, res) => {
       has_more: hasMore,
       last_line: lastLine,
       first_line: firstLine,
+      ...(turnEdits ? { turn_edits: turnEdits } : {}),
     });
   } catch (err) {
     res.json({ messages: [], total: 0, has_more: false, last_line: 0, first_line: 0 });