Add tmux availability check for conditional interactive_bash tool registration

- Implement getTmuxPath() utility to detect tmux availability at plugin load time
- Add getCachedTmuxPath() for retrieving cached tmux path
- Add startBackgroundCheck() for asynchronous tmux detection
- Conditionally register interactive_bash tool only when tmux is available
- Silently skip registration without error messages if tmux not found
- Export utilities from tools/interactive-bash/index.ts

Tool now gracefully handles systems without tmux installed.

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/tools/index.ts

@@ -21,7 +21,9 @@ import { grep } from "./grep"
 import { glob } from "./glob"
 import { slashcommand } from "./slashcommand"
 import { skill } from "./skill"
-import { interactive_bash } from "./interactive-bash"
+
+export { interactive_bash, startBackgroundCheck as startTmuxCheck } from "./interactive-bash"
+export { getTmuxPath } from "./interactive-bash/utils"
 
 import {
   createBackgroundTask,
@@ -63,5 +65,4 @@ export const builtinTools = {
   glob,
   slashcommand,
   skill,
-  interactive_bash,
 }