Init doesn't leave a failed attempt with a non-empty directory

Init can fail which leaves the template files (code, infra, etc.) in the directory, which means you cant easily do another init to recover.

- azd ai agent init creates the target dir and (for container agents) copies/downloads source files in downloadAgentYaml before running the model/deploy/parameter prompts.
- When a later prompt fails, the dir is left non-empty -> a re-run hits resolveCollisions and either prompts to rename or (no-prompt) silently renames to a -2 suffix.

Defer any files until all the info is collected using a stage and commit pattern, similar to azd init.

This can happen when a coding agent tries to --no-prompt, but then doesnt have the subscription info set, so then it will set it and try to `init` again, but the directory has old files in it.