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.

docs/19-http-fallback-transport.md

@@ -305,7 +305,7 @@ New `iris/net/HttpGateway.kt` (OkHttp) + a transport state machine inside
     auth → 401, magic-byte reclassification; `GET /v1/media/{id}` happy path
     (bytes + content-type), unknown id → 404, denied path → 404.
 - **Probe:** `ws_probe.py` gains an `--http` mode (health, post, SSE read with
-  assertion flags, per `gateway-plugin/tests/README.md`) + `--http-media FILE`
+  assertion flags, per `tests/README.md`) + `--http-media FILE`
   (v2: upload round-trip via `POST /v1/media`, exit 23 on rejection).
 - **Kotlin** (`:shared` commonTest): SSE parser (multi-line data, comments,
   `Last-Event-ID` bookkeeping); transport state machine transitions (fake