fix(desktop): timestamp every desktop.log / RECENT LOGS line

rememberLog prepended only '[hermes] ' to each line, so desktop.log and
the in-app RECENT LOGS view carried no timestamps while agent.log and
gateway.log (Python logging) did.

Extract the line format into a small pure helper (desktop-log-line.ts)
and prefix each line with an ISO-8601 UTC timestamp shared per chunk,
matching the Python-side convention. Regression tests assert the shape
contract: timestamp + [hermes] tag + verbatim message. Fixes #84405.

apps/desktop/electron/desktop-log-line.test.ts

@@ -0,0 +1,22 @@
+import { describe, expect, it } from 'vitest'
+
+import { formatDesktopLogLine } from './desktop-log-line'
+
+describe('formatDesktopLogLine', () => {
+  it('prefixes each line with an ISO-8601 timestamp and the hermes tag', () => {
+    const line = formatDesktopLogLine('[boot] Resolving Hermes backend')
+
+    // Shape contract (not a snapshot): every desktop log line starts with
+    // an ISO timestamp so multi-surface logs are chronologically readable.
+    // See #84405.
+    expect(line).toMatch(
+      /^\[\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}Z\] \[hermes\] \[boot\] Resolving Hermes backend$/
+    )
+  })
+
+  it('keeps the message verbatim after the prefix', () => {
+    const line = formatDesktopLogLine('Hermes backend exited (0)')
+
+    expect(line).toMatch(/^\[\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}Z\] \[hermes\] Hermes backend exited \(0\)$/)
+  })
+})

apps/desktop/electron/desktop-log-line.ts

@@ -0,0 +1,19 @@
+/**
+ * Desktop log line formatting shared by every desktop log surface:
+ * `desktop.log`, the in-app "RECENT LOGS" view, and crash forensics.
+ *
+ * Historically each line was prefixed with just `[hermes] `, so lines from
+ * different moments were indistinguishable. Every surface now carries an
+ * ISO-8601 UTC timestamp, matching the Python-side `agent.log` /
+ * `gateway.log` convention (`2026-07-12 16:22:17,540 INFO ...`). See #84405.
+ */
+
+/**
+ * Format one desktop log line with an ISO-8601 UTC timestamp.
+ *
+ * `stamp` defaults to now; callers that batch multiple lines (a single
+ * stdout chunk) pass one shared stamp so the group reads as one event.
+ */
+export function formatDesktopLogLine(text: string, stamp = new Date().toISOString()): string {
+  return `[${stamp}] [hermes] ${text}`
+}

apps/desktop/electron/main.ts

@@ -31,6 +31,7 @@ import {
 import nodePty from 'node-pty'
 
 import { classifyActiveRuntime } from './active-runtime-state'
+import { formatDesktopLogLine } from './desktop-log-line'
 import { stopBackendChild as stopBackendChildImpl, stopBackendTreesForUpdate } from './backend-child'
 import { dashboardFallbackArgs, sourceDeclaresServe } from './backend-command'
 import { createBackendConnectionState } from './backend-connection-state'
@@ -1347,7 +1348,10 @@ function rememberLog(chunk) {
     return
   }
 
-  const lines = text.split(/\r?\n/).map(line => `[hermes] ${line}`)
+  // One timestamp per chunk: lines arriving in the same event happened
+  // at the same moment.  ISO-8601 UTC, matching agent.log/gateway.log.
+  const stamp = new Date().toISOString()
+  const lines = text.split(/\r?\n/).map(line => formatDesktopLogLine(line, stamp))
   hermesLog.push(...lines)
 
   if (hermesLog.length > 300) {