refactor: use ContextCollector for hook injection and remove unused background tools

Split changes:
- Replace injectHookMessage with ContextCollector.register() pattern for improved hook content injection
- Remove unused background task tools infrastructure (createBackgroundOutput, createBackgroundCancel)

🤖 Generated with assistance of OhMyOpenCode (https://github.com/code-yeongyu/oh-my-opencode)

src/tools/index.ts

@@ -35,25 +35,14 @@ export { createSkillTool } from "./skill"
 export { getTmuxPath } from "./interactive-bash/utils"
 export { createSkillMcpTool } from "./skill-mcp"
 
-import {
-  createBackgroundOutput,
-  createBackgroundCancel,
-} from "./background-task"
-
-import type { PluginInput, ToolDefinition } from "@opencode-ai/plugin"
-import type { BackgroundManager } from "../features/background-agent"
-
-type OpencodeClient = PluginInput["client"]
+import type { ToolDefinition } from "@opencode-ai/plugin"
 
 export { createCallOmoAgent } from "./call-omo-agent"
 export { createLookAt } from "./look-at"
 export { createSisyphusTask, type SisyphusTaskToolOptions, DEFAULT_CATEGORIES, CATEGORY_PROMPT_APPENDS } from "./sisyphus-task"
 
-export function createBackgroundTools(manager: BackgroundManager, client: OpencodeClient): Record<string, ToolDefinition> {
-  return {
-    background_output: createBackgroundOutput(manager, client),
-    background_cancel: createBackgroundCancel(manager, client),
-  }
+export function createBackgroundTools(): Record<string, ToolDefinition> {
+  return {}
 }
 
 export const builtinTools: Record<string, ToolDefinition> = {