Implement Custom Atmos Shell Prompts inside Geodesic

[coderabbitai]

As discussed in PR #827, we need to solve for custom Atmos shell prompts inside Geodesic.

atmos/internal/exec/shell_utils.go

Lines 265 to 270 in 73c8c49

	// This means you cannot have a custom shell prompt inside Geodesic (Geodesic requires "-l").
	// Perhaps we should have special detection for Geodesic?
	// We could test if env var GEODESIC_SHELL is set to "true" (or set at all).
	if !hasCustomShellPrompt {
	shellCommand = shellCommand + " -l"
	}

[osterman]

@Nuru what if we added support for custom args to the shell, to make it more configurable:

In atmos.yml we already support a configuration section for the prompt.

components:
  terraform:
    shell:
      prompt: ...
      args: "-l"