fix: skip init prompts when stdin is not a TTY

[manish-wekan]

Summary

• Skip Description/Author prompts in nitrostack-py init when stdin is not a TTY, so NitroStudio generate (step 4 of 5) and CI cannot hang on readline().
• --skip-install still only skips dependency install; this change is the missing non-interactive path.
• If the target directory already exists and stdin is not a TTY, exit 1 instead of prompting to overwrite.
• Add isolated CLI tests that leave stdin open and never write, matching Studio's inherited-stdin hang.

Test plan

• python -m pytest tests/test_cli_studio_noninteractive.py
• python -m nitrostack.cli.main init demo --template python-starter --skip-install with stdin closed completes without prompts
• Interactive TTY init still asks Description/Author
• NitroStudio generate Python project no longer hangs on step 4 of 5

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