1.0.42

fix: Change default backend from mcp to shell in startup-validation.ts

Fixed startup-validation.ts line 447 where backend default was hardcoded to 'mcp' instead of matching DEFAULT_CONFIG.defaultBackend ('shell'). This was causing the validation output to incorrectly show "Backend: mcp" even when no -b flag was provided.

Changes:
- startup-validation.ts line 447: Changed default from 'mcp' to 'shell' with clarifying comment
- startup-validation.ts lines 448-469: Refactored backend detection logic to use backendSetViaCliArg flag for clearer control flow (replaced hardcoded 'mcp' check with explicit flag)
- startup-validation.test.ts lines 417-424: Updated test to mock process.argv with -b mcp flag since shell backend doesn't require mcp.json

All 17 startup-validation tests now pass. CLI correctly shows "Backend: shell (skipping MCP configuration)" when no -b flag is provided.

Resolves task hQI4WX

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Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>