fix: inherit environment variables in LocalShellBackend

[avoidwork]

Description

LocalShellBackend was created without inheritEnv: true, so child processes spawned by the core backend had no environment variables. This meant tools like shell, executeCode, and subagents could not access PATH, HOME, or any other env vars needed to function.

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor (no functional changes)
• Performance improvement
• CI / build / tooling

Testing

• npm run lint passes
• Tests pass with maintained line coverage
• No forbidden patterns used
• Conventional Commit style applied