gemini: Update settings

* Use the latest (20250917) schema:
  https://geminicli.com/docs/get-started/configuration/
* Make `//GEMINI.md` the only always-loaded context file

Change-Id: I9621b88662847b5c531c21bd2aa271743dc30fc1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/352815
Copybara-Verified: Copybara Prod <copybara-worker-blackhole@google.com>
Docs-Not-Needed: Kayce Basques <kayce@google.com>
Reviewed-by: Asad Memon <asadmemon@google.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Commit-Queue: Kayce Basques <kayce@google.com>
diff --git a/.gemini/settings.json b/.gemini/settings.json
index 9445be1..692cd09 100644
--- a/.gemini/settings.json
+++ b/.gemini/settings.json
@@ -1,12 +1,10 @@
 {
-  "contextFileName": [
-    "GEMINI.md",
-    "docs/work_order/work_order.md",
-    "docs/work_order/compile_docs_examples_trigger.md",
-    "docs/work_order/new_feature_trigger.md",
-    "docs/work_order/test_enhancement_audit_trigger.md"
-  ],
-  "fileFiltering": {
-    "respectGitIgnore": true
+  "context": {
+    "fileName": [
+      "GEMINI.md"
+    ],
+    "fileFiltering": {
+      "respectGitIgnore": true
+    }
   }
 }