fix(mcpb): use server/ subdir and ${__dirname} in command per mcpb spec

Official mcpb calculator-rust example uses:
  entry_point: 'server/mcp-calculator'
  command: '${__dirname}/server/mcp-calculator'

${__dirname} is resolved by Claude Code to the extraction directory,
giving an absolute path. This fixes ENOENT on Windows spawn.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Claude Sonnet (coordinator)

xtask/src/bundler.rs

@@ -62,8 +62,8 @@ impl McpbBundler {
         zip.start_file("manifest.json", options)?;
         zip.write_all(manifest_json.as_bytes())?;
 
-        // 2. binary — stored under the canonical entry_point name from the manifest
-        //    (drops .exe suffix on Windows so the ZIP is cross-platform consistent)
+        // 2. binary — stored under server/<entry_point> per mcpb spec
+        //    (${__dirname}/server/<binary> is how Claude Code resolves the absolute path)
         let bin_options = options.unix_permissions(0o755);
         zip.start_file(&self.manifest.server.entry_point, bin_options)?;
         let mut bin_file = fs::File::open(&self.binary_path)?;

xtask/src/main.rs

@@ -168,10 +168,11 @@ fn ledgerr_manifest(version: &str, binary_name: &str) -> McpbManifest {
         },
         server: ManifestServer {
             server_type: ServerType::Binary,
-            entry_point: binary_name.into(),
+            // Binary lives in server/ subdirectory per mcpb spec
+            entry_point: format!("server/{binary_name}"),
             mcp_config: McpConfig {
-                // No ./ prefix — Claude Code resolves the extraction path itself
-                command: binary_name.into(),
+                // ${__dirname} is resolved by Claude Code to the extraction directory
+                command: format!("${{__dirname}}/server/{binary_name}"),
                 args: vec![],
                 env: None,
             },