fix: default non-interactive init to copilot integration

[afurm]

Description

This PR fixes a non-interactive specify init crash path where init would still try to open the interactive integration picker when stdin is not a TTY.
When --ai/--integration is not provided in non-interactive environments, the command now defaults to copilot and proceeds without prompting.

Fixes: github/spec-kit#267

It also adds a regression test to ensure non-interactive initialization:

• does not invoke the integration picker
• writes copilot to .specify/integration.json
• creates expected Copilot scaffold files

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest
  (ran focused pytest targets instead)
• Tested with a sample project (if applicable)

Additional executed checks:

• uv run --with pytest pytest tests/integrations/test_cli.py::TestInitIntegrationFlag::test_noninteractive_init_defaults_to_copilot -q
• uv run --with pytest pytest tests/integrations/test_cli.py -q
• uv run --with pytest pytest tests/integrations/test_integration_copilot.py -q

AI Disclosure

• I did use AI assistance (describe below)

This PR was authored with help from ChatGPT/Codex for issue triage, implementation, test additions, and write-up.

• I did not use AI assistance for this contribution

[Copilot]

Pull request overview

Fixes a non-interactive specify init crash by preventing the interactive integration picker from running when stdin is not a TTY; in that case (and with no --ai/--integration provided) it defaults to the copilot integration and continues. Adds a regression test to ensure non-interactive initialization selects Copilot and produces expected scaffold output.

Changes:

• Default specify init to copilot when sys.stdin.isatty() is false and no agent was explicitly provided.
• Add a CLI-level regression test that monkeypatches the picker to ensure it is not invoked in non-interactive mode.

Show a summary per file

File	Description
src/specify_cli/__init__.py	Adds a non-interactive guard to bypass arrow-key selection and default to copilot.
tests/integrations/test_cli.py	Adds a regression test verifying non-interactive init does not open the picker and writes copilot to .specify/integration.json.

Copilot's findings

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• Files reviewed: 2/2 changed files
• Comments generated: 0