Make --target explicit and prompted if flag is missing

[ejacquier]

Summary

• Removed CRE_TARGET from the generated .env file — target selection is now always explicit
• When --target flag and CRE_TARGET env var are both missing, the CLI now discovers available targets from project.yaml and either:
  • Auto-selects if only one target exists
  • Prompts the user to choose from a styled interactive select menu if multiple targets exist
• GetTarget() no longer returns an error when target is unset — it returns empty string and defers to the new promptForTarget() flow
• Added GetAvailableTargets() to parse project.yaml keys (preserving YAML order) and identify target configurations
• Updated tests to reflect the new behavior (empty target triggers prompt instead of hard error)

Why

Users were confused about which target they were operating against. The old default (CRE_TARGET=staging-settings in .env) silently picked a target without the user's awareness. Making target selection explicit: via flag, env var, or interactive
prompt, ensures users always know which target they're using.