Add automatic runtime_hint setting to publisher create command (#205)

The `publisher create` command now automatically sets the `runtime_hint`
field in the generated `server.json` based on the `registry_name` when
no explicit `--runtime-hint` is provided:

- When `registry_name` is `docker`, sets `runtime_hint` to `docker`
- When `registry_name` is `npm`, sets `runtime_hint` to `npx`
- Other registries remain unchanged (no `runtime_hint` field)

## Before
```bash
# NPM registry - no runtime_hint field generated
$ mcp-publisher create --name "test-server" --description "Test" --repo-url "https://github.com/test/repo" --registry npm
```

```json
{
  "packages": [
    {
      "registry_name": "npm",
      "name": "test-server",
      "version": "1.0.0"
    }
  ]
}
```

## After
```bash
# NPM registry - automatically sets runtime_hint to "npx"
$ mcp-publisher create --name "test-server" --description "Test" --repo-url "https://github.com/test/repo" --registry npm
```

```json
{
  "packages": [
    {
      "registry_name": "npm",
      "name": "test-server",
      "version": "1.0.0",
      "runtime_hint": "npx"
    }
  ]
}
```

The manual `--runtime-hint` flag continues to work and takes precedence
over the automatic setting, ensuring backward compatibility.

Fixes #204.

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tools/publisher/main.go

@@ -263,6 +263,16 @@ func createCommand() error {
 		packageVersion = version
 	}
 
+	// Set runtime hint based on registry name if not explicitly provided
+	if runtimeHint == "" {
+		switch registryName {
+		case "docker":
+			runtimeHint = "docker"
+		case "npm":
+			runtimeHint = "npx"
+		}
+	}
+
 	// Create server structure
 	server := createServerStructure(name, description, version, repoURL, repoSource,
 		registryName, packageName, packageVersion, runtimeHint, execute, envVars, packageArgs)