What does this PR do?

Preserves an explicitly configured HERMES_HOME when generating Linux systemd gateway units, including the common sudo case where the current process environment no longer carries the target user's custom Hermes home.

Related Issue

Fixes #6729
Related to #5947, but intentionally limited to the systemd gateway install path

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

• updated hermes_cli/gateway.py to preserve explicit HERMES_HOME and probe the target user's login environment before falling back to the existing remap/default logic
• preserved the current explicit-env remap behavior for /root/.hermes and /root/.hermes/profiles/<name> sudo cases
• added regression coverage in tests/hermes_cli/test_gateway_service.py for explicit env precedence, probed custom/profile homes, and empty-probe fallback
• kept this branch limited to the systemd install fix instead of bundling separate env/pairing hardening

How to Test

1. Run /Users/kennyxie/Documents/Code/clawspace/hermes-agent/.venv/bin/python -m pytest tests/hermes_cli/test_gateway_service.py -q
2. Run hermes gateway install --system with a non-default HERMES_HOME under sudo and confirm the generated unit uses that home
3. Repeat without an explicit custom home and confirm the existing fallback behavior remains unchanged

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 (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS 26.4

Documentation & Housekeeping

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

Screenshots / Logs

• /Users/kennyxie/Documents/Code/clawspace/hermes-agent/.venv/bin/python -m pytest tests/hermes_cli/test_gateway_service.py -q → 54 passed