fix: subagent inherits parent agent's provider, model, and apiType

Spawned subagents now inherit the parent agent's LLM provider config
(apiType, apiKey, baseURL, model) via ToolContext instead of falling
back to process.env. This fixes failures when the parent agent is
configured via code (not env vars), especially with OpenAI-compatible
endpoints.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/types.ts

@@ -191,6 +191,12 @@ export interface ToolInputSchema {
 export interface ToolContext {
   cwd: string
   abortSignal?: AbortSignal
+  /** Parent agent's LLM provider (inherited by subagents) */
+  provider?: import('./providers/types.js').LLMProvider
+  /** Parent agent's model ID */
+  model?: string
+  /** Parent agent's API type */
+  apiType?: import('./providers/types.js').ApiType
 }
 
 export interface ToolResult {