fix(cmd): ensure GITEA_HOST can be read (#23)

## Why

With the following configuration:

```bash
docker run -i --rm -e GITEA_HOST=<gitea_host> -e GITEA_ACCESS_TOKEN=<gitea_access_token_for_host> docker.gitea.com/gitea-mcp-server:latest
```

after mcp-client calling a tool, the gitea client will encounter the following fatal error:

```
FATAL gitea/gitea.go:47 create gitea client err: user does not exist [uid: 0, name: ]
  gitea.com/gitea/gitea-mcp/pkg/gitea.Client.func1
    /app/pkg/gitea/gitea.go:47
  sync.(*Once).doSlow
    /usr/local/go/src/sync/once.go:78
  sync.(*Once).Do
    /usr/local/go/src/sync/once.go:69
  gitea.com/gitea/gitea-mcp/pkg/gitea.Client
    /app/pkg/gitea/gitea.go:21
  gitea.com/gitea/gitea-mcp/operation/search.SearchReposFn
    /app/operation/search/search.go:161
  github.com/mark3labs/mcp-go/server.(*MCPServer).handleToolCall
    /go/pkg/mod/github.com/mark3labs/mcp-go@v0.18.0/server/server.go:717
  github.com/mark3labs/mcp-go/server.(*MCPServer).HandleMessage
    /go/pkg/mod/github.com/mark3labs/mcp-go@v0.18.0/server/request_handler.go:264
  github.com/mark3labs/mcp-go/server.(*StdioServer).processMessage
    /go/pkg/mod/github.com/mark3labs/mcp-go@v0.18.0/server/stdio.go:228
  github.com/mark3labs/mcp-go/server.(*StdioServer).processInputStream
    /go/pkg/mod/github.com/mark3labs/mcp-go@v0.18.0/server/stdio.go:143
  github.com/mark3labs/mcp-go/server.(*StdioServer).Listen
    /go/pkg/mod/github.com/mark3labs/mcp-go@v0.18.0/server/stdio.go:209
  github.com/mark3labs/mcp-go/server.ServeStdio
    /go/pkg/mod/github.com/mark3labs/mcp-go@v0.18.0/server/stdio.go:282
  gitea.com/gitea/gitea-mcp/operation.Run
    /app/operation/operation.go:48
  gitea.com/gitea/gitea-mcp/cmd.Execute
    /app/cmd/cmd.go:119
  main.main
    /app/main.go:12
  runtime.main
    /usr/local/go/src/runtime/proc.go:283
```

Turns out the root cause was because the `GITEA_HOST` environment variable wasn't overriding the default flag value, resulting in mismatch of host and access token.

The if statement won't be entered
7cfa1fa218/cmd/cmd.go (L74-L77)

Due to `host` could never be evaluated as an empty string from the default value `"http://gitea.com"`
7cfa1fa218/cmd/cmd.go (L35-L40)

Unless user specify `gitea-mcp ... --host <empty_string> ...` with environment `GITEA_HOST=<non_empty_string>` at the same time, which is very unlikely IMHO.

## How

- Set `host` flag default value from `GITEA_HOST` environment variable value
- Remove possible dead code if-statement

Co-authored-by: hiifong <i@hiif.ong>
Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/23
Reviewed-by: hiifong <i@hiif.ong>
Co-authored-by: Flynn Hou <flynnhou7@gmail.com>
Co-committed-by: Flynn Hou <flynnhou7@gmail.com>

cmd/cmd.go

@@ -35,7 +35,7 @@ func init() {
 	flag.StringVar(
 		&host,
 		"host",
-		"https://gitea.com",
+		os.Getenv("GITEA_HOST"),
 		"Gitea host",
 	)
 	flag.IntVar(
@@ -72,9 +72,6 @@ func init() {
 	flag.Parse()
 
 	flagPkg.Host = host
-	if flagPkg.Host == "" {
-		flagPkg.Host = os.Getenv("GITEA_HOST")
-	}
 	if flagPkg.Host == "" {
 		flagPkg.Host = "https://gitea.com"
 	}