fix(openviking): harden local server and .env handling

[ehz0ah]

What does this PR do?

This PR consolidates #78219 and #78940. It fixes two OpenViking setup and startup failures.

First, the Hermes Desktop backend can include the Hermes virtual-environment packages in PYTHONPATH. An auto-started openviking-server inherited this path and could import those packages instead of its own. On Windows, loaded .pyd files could lock that environment and block hermes update.

Second, the OpenViking .env writer used strict UTF-8. A UTF-8 BOM prevented the first key from matching, which could leave a stale duplicate credential. A non-UTF-8 byte could stop setup. The new read path removes a BOM and preserves undecodable bytes when it updates another value.

The original authorship from #78219 and #78940 is preserved. The final follow-up commit changes the original replacement-character behavior so unrelated bytes remain unchanged.

Related Issue

Fixes #78153

Consolidates #78219 and #78940.

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

• Remove PYTHONPATH from the environment of an auto-started OpenViking server. Preserve all other environment variables.
• Read the Hermes .env file with utf-8-sig so a leading BOM does not change the first key.
• Use surrogateescape on read and write so unrelated non-UTF-8 bytes remain unchanged.
• Add regression tests for child environment isolation, BOM handling, byte preservation, and normal UTF-8 updates.

How to Test

1. Run the focused OpenViking tests:

   HERMES_TEST_FILE_RETRIES=0 scripts/run_tests.sh tests/openviking_plugin/ tests/plugins/memory/test_openviking_endpoint_always_blocked.py tests/plugins/memory/test_openviking_provider.py tests/plugins/memory/test_openviking_shutdown.py -q

2. Run Ruff and the Windows footgun scan:

   ruff check .
   python scripts/check-windows-footguns.py --all

3. Confirm that the focused result is 105 passed.

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 26.6 and targeted Windows compatibility checks

The complete macOS suite was started through scripts/run_tests.sh tests/ -q. It cannot be an all-green local gate in the standard development environment because some unrelated tests require optional Teams and WeCom dependencies, and one Linux abstract-socket test runs on macOS. The branch-specific OpenViking suite passes with retries disabled.

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

OpenViking scope: 105 passed, 0 failed
Ruff: All checks passed
Windows footgun scan: No Windows footguns found (973 files scanned)

[ehz0ah]

Updated after the accuracy review:

• Rebasing now uses current upstream main (06b914110).
• The PYTHONPATH comment and PR body now state the verified Desktop behavior: the backend can include the Hermes virtual-environment packages. Fixes #78153 remains because this is the reported process path.
• The duplicate-key comment now states the actual rule: Hermes and python-dotenv use the last occurrence. The test still requires one unambiguous stored value.
• The original functional commits remain unchanged. The explanation corrections are in a separate maintainer-authored commit.
• The general hermes_cli/config.py .env writer remains outside this PR and should be handled separately.

Validation on the rebased branch:

• OpenViking scope: 105 passed, 0 failed
• Ruff: passed
• Windows footgun scan: passed
• git diff --check: passed