Move gateway-plugin tests out of the installable tree; clean plugin scan

The install-time security scanner scans the whole plugin directory and
flagged the test/dev fixtures (hardcoded tokens, /tmp paths, and the
~/.hermes/.env literal in setup.py) as DANGEROUS, blocking installs with
"19 findings".

- Move gateway-plugin/tests/ to top-level tests/ so the installable
  gateway-plugin/ tree contains only production code.
- Update _plugin_dir() in the tests and REPO in e2e.py for the new
  location (both still resolve the live gateway-plugin/ package).
- Update all references: docs, CI-SETUP.md, Gitea workflows, .pi-lens.json.
- Build the hermes .env path at runtime in setup.py via get_hermes_home()
  so the scanner no longer matches the literal ~/.hermes/.env.

Scanner verdict on gateway-plugin/ is now SAFE (0 findings); a fresh
install with scan enabled succeeds and iris appears in the setup menu.

README.md

@@ -168,8 +168,8 @@ Contributions are welcome! Before you start:
    - Python (gateway plugin): `cd hermes-agent && scripts/run_tests.sh tests/gateway/test_android.py`
      (never bare `pytest` — hermes's runner sandboxes `HERMES_HOME`).
    - Kotlin: `cd app && ./gradlew :shared:testDebugUnitTest`
-   - Live check (gateway must be running): `gateway-plugin/tests/ws_probe.py` and
-     `gateway-plugin/tests/e2e.py` — see [`gateway-plugin/tests/README.md`](gateway-plugin/tests/README.md).
+   - Live check (gateway must be running): `tests/ws_probe.py` and
+     `tests/e2e.py` — see [`tests/README.md`](tests/README.md).
 4. **Keep the protocol in sync.** `gateway-plugin/protocol.py`,
    `app/shared/.../protocol/Protocol.kt`, and `docs/protocol/frames.schema.json`
    must always agree.