fix: restore pinned Git in each Windows bootstrap stage

scripts/install.ps1

@@ -443,8 +443,6 @@ function Get-Uv {
 # the same store slot (<store>\git-<version>-<target>\) pm uses. Returns the
 # git.exe path, or $null when no pinned artifact exists for this target.
 function Get-PinnedGit {
-    $existing = Get-Command git -ErrorAction SilentlyContinue
-    if ($existing) { return $existing.Source }  # dev shortcut; fetches nothing
     $target = "win32-$(Get-WindowsArch)"
     $pin = $script:GitPinFiles[$target]
     if (-not $pin) { return $null }
@@ -482,18 +480,14 @@ function Get-PinnedGit {
     return $gitExe
 }
 
-# Ensure a usable git for the rest of the ladder: pinned pm store slot
-# first, then PATH. Returns $true on success.
+# Each -Stage is a new PowerShell process. Restore the pinned pm store Git
+# PATH in every stage that invokes git; never inherit an unpinned system Git.
 function Ensure-Git {
     $g = Get-PinnedGit
     if (-not $g) { return $false }
-    if ($g -ne "git") {
-        # Store-staged git: expose cmd + usr\bin on this process's PATH so
-        # bare `git` works for the rest of the ladder (the same dirs pm's
-        # git package env() composes).
-        $gitEntry = Split-Path (Split-Path $g -Parent) -Parent
-        $env:Path = "$gitEntry\cmd;$gitEntry\usr\bin;$env:Path"
-    }
+    # The same dirs pm's git package env() composes.
+    $gitEntry = Split-Path (Split-Path $g -Parent) -Parent
+    $env:Path = "$gitEntry\cmd;$gitEntry\usr\bin;$env:Path"
     return $true
 }
 
@@ -552,12 +546,13 @@ $Stages = @(
 $Stages += @{ name = "complete"; title = "Finish install"; category = "runtime"; needs_user_input = $false }
 function Stage-Prerequisites {
     if (-not (Ensure-Git)) {
-        Fail "git is required. Install Git for Windows: https://git-scm.com/download/win"
+        Fail "no pinned Git artifact for this Windows architecture"
     }
     Log "prerequisites ok (git)"
 }
 
 function Stage-Repository {
+    if (-not (Ensure-Git)) { Fail "no pinned Git artifact for this Windows architecture" }
     # An interrupted clone from an older installer can leave a .git with no
     # initial commit, where stash/checkout abort ("You do not have the initial
     # commit yet", #40998). Move it aside -- never delete it, it may hold
@@ -886,7 +881,10 @@ function Confirm-DesktopArtifact {
 
 function Stage-Complete {
     $commit = $Commit
-    if (-not $commit) { $commit = Invoke-Native { git -C $InstallDir rev-parse HEAD 2>$null } }
+    if (-not $commit) {
+        if (-not (Ensure-Git)) { Fail "no pinned Git artifact for this Windows architecture" }
+        $commit = Invoke-Native { git -C $InstallDir rev-parse HEAD 2>$null }
+    }
     if ($commit) {
         $marker = [ordered]@{
             schemaVersion = 1

tests/scripts/install/test_install_ps1_staged_git.py

@@ -0,0 +1,60 @@
+"""The bootstrap's separately launched stages must each use PM's pinned Git."""
+
+import json
+import os
+from pathlib import Path
+import shutil
+import subprocess
+
+import pytest
+
+
+INSTALLER = Path(__file__).resolve().parents[3] / "scripts" / "install.ps1"
+
+
+@pytest.mark.platforms("windows")
+def test_stage_processes_restore_pinned_git_and_never_fall_back(tmp_path):
+    powershell = shutil.which("powershell.exe")
+    system_git = shutil.which("git")
+    assert powershell and system_git
+
+    origin = tmp_path / "origin"
+    origin.mkdir()
+    subprocess.run([system_git, "-C", str(origin), "init", "-q", "-b", "main"], check=True)
+    (origin / "README").write_text("fixture", encoding="utf-8")
+    subprocess.run([system_git, "-C", str(origin), "add", "README"], check=True)
+    subprocess.run([system_git, "-C", str(origin), "-c", "user.name=Smoke",
+                    "-c", "user.email=smoke@example.invalid", "commit", "-qm", "initial"], check=True)
+    expected = subprocess.run([system_git, "-C", str(origin), "rev-parse", "HEAD"],
+                              check=True, capture_output=True, text=True).stdout.strip()
+
+    home = tmp_path / "home"
+    store = tmp_path / "tools"
+    env = dict(os.environ, HERMES_HOME=str(home), HERMES_RUNTIME_DIR=str(store),
+               HERMES_REPO_URL=str(origin))
+
+    def stage(name):
+        result = subprocess.run([powershell, "-NoProfile", "-NonInteractive", "-ExecutionPolicy", "Bypass",
+                                 "-File", str(INSTALLER), "-Stage", name, "-Json"],
+                                env=env, capture_output=True, text=True, timeout=240)
+        frames = [json.loads(line) for line in result.stdout.splitlines() if line.startswith("{")]
+        assert result.returncode == 0 and len(frames) == 1 and frames[0]["ok"], result.stdout + result.stderr
+
+    stage("prerequisites")
+    staged = list(store.glob("git-*/cmd/git.exe"))
+    assert len(staged) == 1
+    stage("repository")  # new process; prerequisites' PATH cannot propagate
+    stage("complete")  # the marker's bare git call is also a new process
+    checkout = home / "hermes-agent"
+    actual = subprocess.run([str(staged[0]), "-C", str(checkout), "rev-parse", "HEAD"],
+                            check=True, capture_output=True, text=True).stdout.strip()
+    marker = json.loads((checkout / ".hermes-bootstrap-complete").read_text(encoding="utf-8-sig"))
+    assert actual == expected == marker["pinnedCommit"]
+
+    # Even when system Git is on PATH, an unsupported pin must fail closed.
+    probe = (f". '{INSTALLER}'; "
+             "$script:GitPinFiles.Remove(('win32-' + (Get-WindowsArch))); "
+             "if (Ensure-Git) { exit 1 } else { exit 0 }")
+    refused = subprocess.run([powershell, "-NoProfile", "-NonInteractive", "-ExecutionPolicy", "Bypass",
+                              "-Command", probe], env=env, capture_output=True, text=True, timeout=30)
+    assert refused.returncode == 0, refused.stdout + refused.stderr