flow init: add non-interactive flag for minimal setup

[jribbink]

Feature request: Add non-interactive flag to flow init for minimal setup

Context:

• Discussed in PR Resolve directories passed to flow init #2055
• Goal: enable CI and scripted init without prompts, accepting safe defaults

Proposal:

• New flag: --non-interactive (aliases: --yes, -y)
• Behavior:
  • Skip all prompts and use defaults for minimal project
  • Do not open editors or interactive selectors
  • Return non-zero exit on missing required inputs

Acceptance criteria:

• Running flow init --non-interactive . completes with no prompts
• Minimal files created as with default choices
• Documented in help and README

Open questions:

• Exact flag name and aliases
• Whether to support --minimal as a separate shortcut