fix: address review findings on SDK godoc and nil guard

- pkg/kit: remove internal package paths from exported godoc on
  ParseTemplate and the ToolKind* constants (SDK doc surface must not
  reference internal packages)
- internal/tools: guard marshalToolResult against a nil CallToolResult
  (json.Marshal(nil) succeeds as 'null', then result.IsError panics if
  a client returns nil result with nil error)

Skipped the TreeNode Children deep-copy suggestion: the slice already
comes from TreeManager.GetChildren which returns a fresh copy per call
into a throwaway intermediate, so no internal state is exposed.

internal/tools/mcp.go

@@ -737,6 +737,9 @@ func (m *MCPToolManager) withOAuthRetry(ctx context.Context, serverName, toolNam
 // marshalToolResult converts an MCP CallToolResult into the JSON-encoded
 // MCPToolResult shape returned to the agent.
 func marshalToolResult(result *mcp.CallToolResult) (*MCPToolResult, error) {
+	if result == nil {
+		return nil, errors.New("mcp tool call returned nil result")
+	}
 	marshaled, err := json.Marshal(result)
 	if err != nil {
 		return nil, fmt.Errorf("failed to marshal mcp tool result: %w", err)

pkg/kit/events.go

@@ -106,8 +106,8 @@ type Event interface {
 // for execute tools) and file trackers to identify which results contain
 // modifications.
 //
-// The canonical classification lives in internal/extensions; these constants
-// re-export it so SDK events and extension events always agree.
+// These constants re-export the canonical classification used by extension
+// events, so SDK events and extension events always agree.
 const (
 	ToolKindExecute  = extensions.ToolKindExecute  // Shell execution (bash)
 	ToolKindEdit     = extensions.ToolKindEdit     // File modification (edit, write)

pkg/kit/template_bridge.go

@@ -16,7 +16,8 @@ import (
 // ---------------------------------------------------------------------------
 
 // ParseTemplate extracts {{variables}} from template content. The template
-// grammar is shared with skill prompt templates (see internal/skills).
+// grammar is shared with skill prompt templates, so a template parses
+// identically regardless of which API loads it.
 func ParseTemplate(name, content string) extensions.PromptTemplate {
 	tpl := skills.NewPromptTemplate(name, content)
 	vars := tpl.Variables