fix(install): reset tree on conflicting stash apply so Hermes stays runnable

[xxxigm]

Summary

The repository stage of scripts/install.sh and scripts/install.ps1 stashes
local changes, pulls, then git stash applys the user's work back on top. When
that apply conflicts, git writes <<<<<<< Updated upstream /
>>>>>>> Stashed changes markers into the tracked source files.

Both shell installers left those markers behind on a conflicting apply
(install.sh exit 1, install.ps1 throw), bricking the checkout. A
poisoned toolsets.py then raises SyntaxError on import and the gateway
never starts — the exact symptom users hit after switching install methods:

File "...\toolsets.py", line 1
    <<<<<<< Updated upstream
    ^^
SyntaxError: invalid syntax

The hermes update Python path (_restore_stashed_changes) already guards
this: on a conflicting apply it git reset --hard HEAD to the freshly-pulled
tree and keeps the stash for manual recovery. This brings the two shell
installers to parity — fixing the whole bug class across the sibling install
paths.

Changes

• scripts/install.sh / scripts/install.ps1: after git stash apply,
  detect a conflict via a non-zero exit or unmerged paths
  (git diff --name-only --diff-filter=U). On conflict, git reset --hard HEAD to restore a clean, runnable tree and keep the stash (never
  silently drop the user's work, per the fix(update): stop stash/restore from clobbering desktop source on managed clones #38542 data-loss class). The code
  update itself succeeded, so the install now completes with a runnable Hermes
  instead of aborting on a poisoned tree.
• install.sh captures the apply exit code with || apply_rc=$? so a failing
  apply doesn't trip set -e before the cleanup runs.
• tests/test_install_stash_apply_conflict.py: reproduces the conflicting
  apply end-to-end and asserts the tree self-heals (no markers, content matches
  HEAD, stash preserved, install doesn't abort), plus the unchanged clean-apply
  happy path and an install.ps1 source-contract check.

Verification

scripts/run_tests.sh tests/test_install_stash_apply_conflict.py \
  tests/test_install_unmerged_index.py \
  tests/hermes_cli/test_update_autostash.py \
  tests/hermes_cli/test_update_post_pull_syntax_guard.py
# 47 passed
bash -n scripts/install.sh   # syntax OK

Scope

Narrowly targets the conflicting-git stash apply-leaves-markers gap in the
shell installers. Complementary to #47373 (recovery-marker / ref-verify
hardening), which does not clean the poisoned working tree on a conflicting
apply. Does not attempt the broader shared-resolver refactor, so it
intentionally does not auto-close #46791.

Refs #46791