Use golangci-lint fmt to format code (#178)

Use `golangci-lint fmt` to format code, replacing the previous gofumpt-based formatter. https://github.com/daixiang0/gci is used to order the imports.

Mirrors https://github.com/go-gitea/gitea/pull/37194.

---
This PR was written with the help of Claude Opus 4.7

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/178
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

.golangci.yml

@@ -102,9 +102,16 @@ issues:
   max-same-issues: 0
 formatters:
   enable:
-    - gofmt
+    - gci
     - gofumpt
   settings:
+    gci:
+      custom-order: true
+      sections:
+        - standard
+        - prefix(gitea.com/gitea/gitea-mcp)
+        - blank
+        - default
     gofumpt:
       extra-rules: true
   exclusions: