fix: prompt before executing test/lint commands from repo config (issue #5254)

[chrislazar25]

Summary: Fix arbitrary command execution at startup when a repository's .aider.conf.yml sets test-cmd or lint-cmd by prompting the user for confirmation before executing.

Technical Details:

• Root Cause: default_config_files includes the git-repo-root .aider.conf.yml. When that file sets test: true + test-cmd, the command executes at startup with shell=True and no confirmation. Attackers can craft a repo that executes arbitrary code the moment aider runs inside the clone.
• Mechanism: Add get_repo_config_cmd_files() that scans config files (excluding user's ~/.aider.conf.yml) for test-cmd, lint-cmd, test:, lint: keys. Before executing test/lint, if dangerous keys are found in repo config and --yes-always was not passed, the user is prompted. If declined, execution returns 0.

Verification: Confirmed adherence to CONTRIBUTING.md. Ran python -m pytest tests/. 455 passed, 1 pre-existing env failure (test_voice requires audio hardware). Pre-commit hooks (isort, black, flake8, codespell) all green. Manual E2E:

• Repo config with test-cmd → correctly detected as dangerous
• Repo config with echo: true only → not flagged
• Home config with test-cmd → excluded from scan
• CLI --test-cmd → no prompt (trusted source)

Blast Radius:

• Impact: Medium (security fix). Changes behavior when a repo-root .aider.conf.yml contains test/lint command keys those now require confirmation before executing.
• Trade-offs: Adds one interactive prompt at startup for affected configs. No impact on CLI-provided test/lint commands or home config. --yes-always skips the prompt.

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Fixes #5254