Improve error handling and display

[vdemeester]

Summary

• Silence Cobra's default error output and use styled logger instead
• Strip Dagger traceparent spans from error messages for cleaner output
• Show error details inline below failed steps and tasks

Before/After

Before:

X Step: init [50.1s] ERROR
Error: execution failed: ... [traceparent:abc123-def456]

After:

X Step: init [50.1s] ERROR
  → failed to resolve image "alpine:latest"
[ERROR] execution failed: failed to resolve image "alpine:latest"

Changes

• cmd/chisel/run.go: Add SilenceErrors: true, use styled fail() helper
• pkg/ui/errors.go: New CleanError/CleanErrorMessage helpers to strip Dagger traces
• pkg/ui/logger.go: Show error message below failed steps/tasks

Test plan

• Unit tests for CleanErrorMessage (8 tests)
• Unit tests for error display in logger (3 tests)
• All existing tests pass

🤖 Generated with Claude Code