[log] Add debug logging to writeGatewayConfig in cmd/root.go (#1533)

## Summary

Adds 5 meaningful `debugLog.Printf` calls to the `writeGatewayConfig`
function in `internal/cmd/root.go` to improve observability when
troubleshooting gateway configuration output issues.

## Changes

The `writeGatewayConfig` function previously had **zero** debug logging
calls despite being the key function that builds and writes the
gateway's output configuration (per MCP Gateway Spec §5.4). When clients
receive unexpected config output, there was no way to trace what the
function was computing.

**Added logging at:**
1. **Function entry** — logs `listenAddr`, `mode`, and total server
count
2. **After address parsing** — logs resolved `host` and `port` (helps
debug IPv6/IPv4 detection)
3. **Auth config** — logs whether an API key is configured (without
exposing the key)
4. **Per-server URL** — logs server name, computed URL, and tool count
5. **Successful completion** — logs final server count after encoding

## Quality Checklist

- [x] Exactly 1 file modified
- [x] No test files modified
- [x] Reuses existing `debugLog` logger (already declared as `var
debugLog = logger.New("cmd:root")`)
- [x] Logger naming follows `pkg:filename` convention (`cmd:root`)
- [x] Logger arguments don't compute anything or cause side effects
(`len()` is a builtin, no function calls)
- [x] Logging messages are meaningful and helpful for debugging
- [x] No duplication with existing logs

## Example Debug Output

```
DEBUG=cmd:root ./awmg --config config.toml
cmd:root Writing gateway config: listenAddr=127.0.0.1:3000, mode=routed, serverCount=2
cmd:root Parsed listen address: host=127.0.0.1, port=3000
cmd:root Gateway config: auth_enabled=true
cmd:root Writing server config: name=github, url=(127.0.0.1/redacted) toolCount=0
cmd:root Writing server config: name=slack, url=(127.0.0.1/redacted) toolCount=0
cmd:root Gateway config written successfully: serverCount=2
```




> Generated by [Go Logger
Enhancement](https://github.com/github/gh-aw-mcpg/actions/runs/22546627400)

<!-- gh-aw-agentic-workflow: Go Logger Enhancement, engine: copilot, id:
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Author: lpcox

internal/cmd/root.go

@@ -341,6 +341,8 @@ func writeGatewayConfigToStdout(cfg *config.Config, listenAddr, mode string) err
 }
 
 func writeGatewayConfig(cfg *config.Config, listenAddr, mode string, w io.Writer) error {
+	debugLog.Printf("Writing gateway config: listenAddr=%s, mode=%s, serverCount=%d", listenAddr, mode, len(cfg.Servers))
+
 	// Parse listen address to extract host and port
 	// Use net.SplitHostPort which properly handles both IPv4 and IPv6 addresses
 	host, port := DefaultListenIPv4, DefaultListenPort
@@ -352,6 +354,7 @@ func writeGatewayConfig(cfg *config.Config, listenAddr, mode string, w io.Writer
 			port = p
 		}
 	}
+	debugLog.Printf("Parsed listen address: host=%s, port=%s", host, port)
 
 	// Determine domain (use host from listen address)
 	domain := host
@@ -361,6 +364,7 @@ func writeGatewayConfig(cfg *config.Config, listenAddr, mode string, w io.Writer
 	if cfg.Gateway != nil {
 		apiKey = cfg.Gateway.APIKey
 	}
+	debugLog.Printf("Gateway config: auth_enabled=%v", apiKey != "")
 
 	// Build output configuration
 	outputConfig := map[string]interface{}{
@@ -374,12 +378,15 @@ func writeGatewayConfig(cfg *config.Config, listenAddr, mode string, w io.Writer
 			"type": "http",
 		}
 
+		var serverURL string
 		if mode == "routed" {
-			serverConfig["url"] = fmt.Sprintf("http://%s:%s/mcp/%s", domain, port, name)
+			serverURL = fmt.Sprintf("http://%s:%s/mcp/%s", domain, port, name)
 		} else {
 			// Unified mode - all servers use /mcp endpoint
-			serverConfig["url"] = fmt.Sprintf("http://%s:%s/mcp", domain, port)
+			serverURL = fmt.Sprintf("http://%s:%s/mcp", domain, port)
 		}
+		serverConfig["url"] = serverURL
+		debugLog.Printf("Writing server config: name=%s, url=%s, toolCount=%d", name, serverURL, len(server.Tools))
 
 		// Add auth headers per MCP Gateway Specification Section 5.4
 		// Authorization header contains API key directly (not Bearer scheme per spec 7.1)
@@ -404,6 +411,7 @@ func writeGatewayConfig(cfg *config.Config, listenAddr, mode string, w io.Writer
 	if err := encoder.Encode(outputConfig); err != nil {
 		return fmt.Errorf("failed to encode configuration: %w", err)
 	}
+	debugLog.Printf("Gateway config written successfully: serverCount=%d", len(servers))
 
 	// Flush stdout buffer if it's a regular file
 	// Note: Sync() fails on pipes and character devices like /dev/stdout,