Update build configuration and improve token handling

- Change CI container image to debian:trixie-slim and set GOPROXY.
- Update Go version to 1.26 in Dockerfile and go.mod.
- Refactor token validation to use SHA-256 hashes instead of plain tokens.
- Add network policy to restrict access to the service.
- Enhance README with new configuration details and usage examples.
- Add tests for new token hash validation logic.

deploy/k3s/deployment.yaml

@@ -4,6 +4,8 @@ metadata:
   name: gitea-mcp-auth-proxy
   labels:
     app: gitea-mcp-auth-proxy
+  annotations:
+    security.note/healthz: "Do not expose /healthz publicly; keep service internal and restrict ingress at network/ingress layer."
 spec:
   replicas: 1
   selector:
@@ -24,7 +26,7 @@ spec:
           env:
             - name: AUTH_PROXY_LISTEN_ADDR
               value: ":8080"
-            - name: AUTH_PROXY_TOKENS_DIR
+            - name: AUTH_PROXY_TOKEN_HASHES_DIR
               value: /var/run/secrets/auth-proxy
             - name: AUTH_PROXY_LOG_LEVEL
               value: info