chore: add golangci-lint, bump Go to 1.26, fix all lint issues (#133)

## Summary
- Add `.golangci.yml` with linter configuration matching the main gitea repo
- Add `lint`, `lint-fix`, `lint-go`, `lint-go-fix`, and `security-check` Makefile targets
- Add `tidy` Makefile target (extracts min Go version from `go.mod` for `-compat` flag)
- Bump minimum Go version to 1.26
- Update golangci-lint to v2.10.1
- Replace `golang/govulncheck-action` with `make security-check` in CI
- Add `make lint` step to CI
- Fix all lint issues across the codebase (formatting, `errors.New` vs `fmt.Errorf`, `any` vs `interface{}`, unused returns, stuttering names, Go 1.26 `new(expr)`, etc.)
- Remove unused `pkg/ptr` package (inlined by Go 1.26 `new(expr)`)
- Remove dead linter exclusions (staticcheck, gocritic, testifylint, dupl)

## Test plan
- [x] `make lint` passes
- [x] `go test ./...` passes
- [x] `make build` succeeds

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/133
Reviewed-by: techknowlogick <techknowlogick@noreply.gitea.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

operation/repo/tag.go

@@ -2,6 +2,7 @@ package repo
 
 import (
 	"context"
+	"errors"
 	"fmt"
 
 	"gitea.com/gitea/gitea-mcp/pkg/gitea"
@@ -89,15 +90,15 @@ func CreateTagFn(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToolRes
 	log.Debugf("Called CreateTagFn")
 	owner, ok := req.GetArguments()["owner"].(string)
 	if !ok {
-		return nil, fmt.Errorf("owner is required")
+		return nil, errors.New("owner is required")
 	}
 	repo, ok := req.GetArguments()["repo"].(string)
 	if !ok {
-		return nil, fmt.Errorf("repo is required")
+		return nil, errors.New("repo is required")
 	}
 	tagName, ok := req.GetArguments()["tag_name"].(string)
 	if !ok {
-		return nil, fmt.Errorf("tag_name is required")
+		return nil, errors.New("tag_name is required")
 	}
 	target, _ := req.GetArguments()["target"].(string)
 	message, _ := req.GetArguments()["message"].(string)
@@ -122,15 +123,15 @@ func DeleteTagFn(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToolRes
 	log.Debugf("Called DeleteTagFn")
 	owner, ok := req.GetArguments()["owner"].(string)
 	if !ok {
-		return nil, fmt.Errorf("owner is required")
+		return nil, errors.New("owner is required")
 	}
 	repo, ok := req.GetArguments()["repo"].(string)
 	if !ok {
-		return nil, fmt.Errorf("repo is required")
+		return nil, errors.New("repo is required")
 	}
 	tagName, ok := req.GetArguments()["tag_name"].(string)
 	if !ok {
-		return nil, fmt.Errorf("tag_name is required")
+		return nil, errors.New("tag_name is required")
 	}
 
 	client, err := gitea.ClientFromContext(ctx)
@@ -149,15 +150,15 @@ func GetTagFn(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToolResult
 	log.Debugf("Called GetTagFn")
 	owner, ok := req.GetArguments()["owner"].(string)
 	if !ok {
-		return nil, fmt.Errorf("owner is required")
+		return nil, errors.New("owner is required")
 	}
 	repo, ok := req.GetArguments()["repo"].(string)
 	if !ok {
-		return nil, fmt.Errorf("repo is required")
+		return nil, errors.New("repo is required")
 	}
 	tagName, ok := req.GetArguments()["tag_name"].(string)
 	if !ok {
-		return nil, fmt.Errorf("tag_name is required")
+		return nil, errors.New("tag_name is required")
 	}
 
 	client, err := gitea.ClientFromContext(ctx)
@@ -176,11 +177,11 @@ func ListTagsFn(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToolResu
 	log.Debugf("Called ListTagsFn")
 	owner, ok := req.GetArguments()["owner"].(string)
 	if !ok {
-		return nil, fmt.Errorf("owner is required")
+		return nil, errors.New("owner is required")
 	}
 	repo, ok := req.GetArguments()["repo"].(string)
 	if !ok {
-		return nil, fmt.Errorf("repo is required")
+		return nil, errors.New("repo is required")
 	}
 	page, _ := req.GetArguments()["page"].(float64)
 	pageSize, _ := req.GetArguments()["pageSize"].(float64)