fix(desktop): bootstrap falls back to installed agent install.sh on GitHub 404

Packaged Desktop first-launch bootstrap no longer dies with a fatal HTTP
404 when install-stamp.json pins a commit that isn't fetchable from GitHub.

This only happens for locally-built desktop apps: write-build-stamp.cjs's
fromLocalGit() pins `git rev-parse HEAD`, which can be an unpushed commit
or dirty tree. CI builds stamp $GITHUB_SHA and are unaffected. The fix
unblocks the dev / self-builder workflow.

resolveInstallScript() now wraps the GitHub download in try/catch; on
failure it resolves ~/.hermes/hermes-agent/scripts/install.sh (the
already-installed agent checkout), copies it into bootstrap-cache, and
returns it as source 'installed-agent'. If the cache copy fails (read-only
FS), it uses the source path directly. With no installed checkout to fall
back to, the original error rethrows unchanged.

Download is now injectable via an optional _download param so the fallback
path is tested hermetically (no network).

Reported with a precise repro and suggested fix by @Tamaz-sujashvili (#40815).

Co-authored-by: Tamaz-sujashvili <56168197+Tamaz-sujashvili@users.noreply.github.com>

apps/desktop/electron/bootstrap-runner.cjs

@@ -76,6 +76,21 @@ function bootstrapCacheDir(hermesHome) {
   return path.join(hermesHome, 'bootstrap-cache')
 }
 
+// The install.sh / install.ps1 that ships inside the already-installed agent
+// checkout under ~/.hermes/hermes-agent. Used as a last-resort fallback when
+// the pinned commit can't be fetched from GitHub (e.g. a locally-built desktop
+// app stamped to an unpushed HEAD).
+function installedAgentInstallScript(hermesHome) {
+  if (!hermesHome) return null
+  const candidate = path.join(hermesHome, 'hermes-agent', 'scripts', installScriptName())
+  try {
+    fs.accessSync(candidate, fs.constants.R_OK)
+    return candidate
+  } catch {
+    return null
+  }
+}
+
 function cachedScriptPath(hermesHome, commit) {
   return path.join(bootstrapCacheDir(hermesHome), `install-${commit}.${process.platform === 'win32' ? 'ps1' : 'sh'}`)
 }
@@ -155,7 +170,7 @@ function downloadInstallScript(commit, destPath) {
   })
 }
 
-async function resolveInstallScript({ installStamp, sourceRepoRoot, hermesHome, emit }) {
+async function resolveInstallScript({ installStamp, sourceRepoRoot, hermesHome, emit, _download = downloadInstallScript }) {
   // 1. Dev shortcut: prefer a local checkout's installer so we can iterate
   //    without pushing. SOURCE_REPO_ROOT comes from main.cjs (path.resolve
   //    of APP_ROOT/../..).
@@ -189,9 +204,35 @@ async function resolveInstallScript({ installStamp, sourceRepoRoot, hermesHome,
     type: 'log',
     line: `[bootstrap] fetching ${installScriptName()} for ${installStamp.commit.slice(0, 12)} from GitHub`
   })
-  await downloadInstallScript(installStamp.commit, cached)
-  emit({ type: 'log', line: `[bootstrap] saved to ${cached}` })
-  return { path: cached, source: 'download', commit: installStamp.commit, kind: installScriptKind() }
+  try {
+    await _download(installStamp.commit, cached)
+    emit({ type: 'log', line: `[bootstrap] saved to ${cached}` })
+    return { path: cached, source: 'download', commit: installStamp.commit, kind: installScriptKind() }
+  } catch (err) {
+    // The pinned commit may not be fetchable from GitHub -- most commonly a
+    // locally-built desktop app stamped to an unpushed HEAD (see
+    // write-build-stamp.cjs fromLocalGit). Fall back to the installer that
+    // ships inside the already-installed agent checkout so dev/self-builds can
+    // still bootstrap instead of dying with a fatal 404.
+    const installed = installedAgentInstallScript(hermesHome)
+    if (installed) {
+      emit({
+        type: 'log',
+        line:
+          `[bootstrap] GitHub fetch failed (${err.message}); ` +
+          `falling back to installed agent ${installScriptName()} at ${installed}`
+      })
+      try {
+        fs.mkdirSync(path.dirname(cached), { recursive: true })
+        fs.copyFileSync(installed, cached)
+        return { path: cached, source: 'installed-agent', commit: installStamp.commit, kind: installScriptKind() }
+      } catch {
+        // Cache copy failed (read-only FS, etc.) -- use the source path directly.
+        return { path: installed, source: 'installed-agent', commit: installStamp.commit, kind: installScriptKind() }
+      }
+    }
+    throw err
+  }
 }
 
 // ---------------------------------------------------------------------------
@@ -673,5 +714,7 @@ module.exports = {
   // Exposed for testability
   parseStageResult,
   resolveLocalInstallScript,
+  resolveInstallScript,
+  installedAgentInstallScript,
   cachedScriptPath
 }