What does this PR do?

Fixes the macOS LaunchAgent reinstall path so hermes gateway install no longer silently resets an existing service from a custom HERMES_HOME back to the current shell/default Hermes home.

The fix reads service-scoped values from the existing plist before regenerating it, then preserves:

• EnvironmentVariables.HERMES_HOME
• EnvironmentVariables.PATH
• EnvironmentVariables.VIRTUAL_ENV
• StandardOutPath
• StandardErrorPath

This keeps the repair/reinstall behavior intact while preventing an existing custom gateway home, path, venv, or log target from being overwritten by the caller's current environment. The stale-plist comparison also uses those preserved values so a valid custom-home plist does not keep looking outdated forever.

Related Issue

Fixes #32741

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• hermes_cli/gateway.py: add plist parsing for existing launchd service values and preserve those values during plist comparison, refresh, and force reinstall.
• tests/hermes_cli/test_gateway_service.py: add regression coverage for reinstalling an outdated LaunchAgent plist while preserving custom HERMES_HOME, PATH, VIRTUAL_ENV, stdout log path, and stderr log path.

How to Test

1. Install gateway once with a custom HERMES_HOME.
2. Run hermes gateway install later from an environment where HERMES_HOME is unset or different.
3. Verify the installed plist still points at the original custom service values.

Automated verification run locally:

scripts/run_tests.sh tests/hermes_cli/test_gateway_service.py -- -q -k launchd
uv run ruff check .
python scripts/check-windows-footguns.py --all
git diff --check

Results:

• launchd slice: 14 passed
• ruff: passed
• Windows footgun scan: passed
• whitespace check: passed

I also tried the full tests/hermes_cli/test_gateway_service.py file on macOS. The launchd tests passed, but six unrelated systemd tests fail on current upstream/main before this patch's launchd path because _preflight_user_systemd() rejects the local non-Linux D-Bus/systemd environment.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes
• I've tested on my platform: macOS

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — N/A
• I've updated cli-config.yaml.example if I added/changed config keys — N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — N/A
• I've considered cross-platform impact per the compatibility guide
• I've updated tool descriptions/schemas if I changed tool behavior — N/A

Screenshots / Logs

Not applicable. This is covered by launchd plist regression tests.