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>

pkg/gitea/gitea.go

@@ -34,7 +34,7 @@ func NewClient(token string) (*gitea.Client, error) {
 	}
 
 	// Set user agent for the client
-	client.SetUserAgent(fmt.Sprintf("gitea-mcp-server/%s", flag.Version))
+	client.SetUserAgent("gitea-mcp-server/" + flag.Version)
 	return client, nil
 }