fix(ci): install golangci-lint v2.1.6 manually to match local environment

Install v2.1.6 manually in CI (not via action) to avoid --out-format flag
compatibility issues. This ensures both local and CI use the same version
and support version: 2 config format.

.golangci.yml

@@ -1,7 +1,7 @@
 # golangci-lint configuration
 # See https://golangci-lint.run/usage/configuration/
-# Note: No version field - compatible with both v1.64.8 (CI) and v2.x (local)
-# Local v2.x may show a deprecation warning but will work
+
+version: 2
 
 run:
   timeout: 5m