refactor: decompose bash tool into helper functions

Extracted three helper functions to improve clarity and reduce duplication:

1. **validateScript()** - Centralizes all input validation (empty script, sleep
   commands, redundant cd). Returns error result or null.

2. **createLineHandler()** - Unified line processing logic for both stdout and
   stderr. Eliminates 84 lines of duplicate code. Enforces truncation limits
   consistently.

3. **formatResult()** - Handles all result formatting based on exit code and
   truncation state. Simplifies the main execution flow.

## Benefits

- **-80 LoC of duplication removed** (stdout/stderr handlers)
- **Clearer separation of concerns** - validation, processing, formatting
- **Easier to test** - helpers are pure functions
- **More maintainable** - single source of truth for line processing logic

## Testing

- All 955 unit tests pass
- Static checks pass (lint + typecheck + format)
- No behavior changes, pure refactor

---
_Generated with `cmux`_

Author: ammar-agent