fix(history): raise the transcript head-read window to fit restart bookkeeping

Blank firstPrompt rows weren't all oversized messages -- traced one
directly: a session restarted many times (mux deaths, redeploys)
accumulates a batch of small bookkeeping lines (mode/permission-mode/
last-prompt/queue-operation, one batch per restart) ahead of the real
first message. With enough restarts these alone crossed the old 16KB
head-read window, so extraction found nothing even though the actual
first message was tiny (measured case: ~17.5KB of bookkeeping pushed a
189-byte real message just past the boundary).

Raise the head buffer from 16KB to 128KB (matching the existing
precedent at the codex-history head-read a few hundred lines up) and
fix three now-stale `> 16384`/`> 65536` fallback thresholds to
reference headBuf.length instead of hardcoded numbers, so the tail-read
fallbacks stay correctly scoped to "beyond what head already covered."

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

test/routes/session-routes.test.ts

@@ -1395,6 +1395,40 @@ describe('session-routes', () => {
       expect(ids).toContain(noEntrypointId);
       expect(ids).not.toContain(sdkId);
     });
+
+    it('finds the real first prompt past a large run of pre-message bookkeeping lines', async () => {
+      // A session restarted many times over a long conversation accumulates a batch
+      // of small bookkeeping lines (mode/permission-mode/last-prompt/queue-operation)
+      // per restart, ahead of the real first message. With enough restarts these can
+      // push the genuine first prompt past a 16KB head-read window even though the
+      // message itself is tiny — the row showed up blank despite having real content.
+      const home = process.env.HOME as string;
+      const projPath = join(home, '.claude', 'projects', 'proj-bookkeeping-test');
+      await mkdir(projPath, { recursive: true });
+
+      const sessionId = '66666666-6666-6666-6666-666666666666';
+      const bookkeepingLine = JSON.stringify({ type: 'mode', mode: 'normal', sessionId }) + '\n';
+      // > 16KB (the old head-read size) but well under 128KB (the new one).
+      const prefix = bookkeepingLine.repeat(Math.ceil(20000 / bookkeepingLine.length));
+      const realLine =
+        JSON.stringify({
+          type: 'user',
+          entrypoint: 'cli',
+          message: { role: 'user', content: 'the real first message' },
+        }) + '\n';
+      expect(prefix.length).toBeGreaterThan(16384);
+
+      await writeFile(join(projPath, `${sessionId}.jsonl`), prefix + realLine);
+
+      const res = await harness.app.inject({
+        method: 'GET',
+        url: '/api/history/sessions?projectKey=proj-bookkeeping-test',
+      });
+      expect(res.statusCode).toBe(200);
+      const row = JSON.parse(res.body).data.sessions.find((s: { sessionId: string }) => s.sessionId === sessionId);
+      expect(row).toBeDefined();
+      expect(row.firstPrompt).toBe('the real first message');
+    });
   });
 
   // ========== POST /api/sessions (with resumeSessionId) ==========