Add preflight checks and prompt file input support

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Summary

This PR introduces a preflight check mechanism for the CLI that verifies, before starting the agent, that:

• The specified repo_path is inside a Git worktree.
• The node binary is on PATH (and its version is logged).
• The docker binary is on PATH (and its version is logged).

In addition, it implements support for reading a prompt from a file when --prompt is given a filepath.

Implementation Details

• Created src/oai_coding_agent/preflight.py with:
  • is_inside_git_repo(), get_tool_version(), check_node(), check_docker() helpers.
  • run_preflight_checks() orchestrator that prints errors via typer.echo and exits with typer.Exit(code=1) if any check fails.
• Integrated run_preflight_checks() into cli.main() immediately after building the Config.
• Extended the CLI prompt logic to detect when --prompt points to a file and load its contents.
• Logged Node.js and Docker versions at INFO level using the existing logger, so they are written to ~/.oai_coding_agent/agent.log.

Assumptions

• The Git binary is required to determine worktree membership; if Git is not installed, the check will fail with a clear error.
• For simplicity, version commands use node --version and docker --version.

Testing

• New unit tests in tests/test_preflight.py cover successful and failure scenarios for preflight checks (Git, Node.js, Docker) and validate the version logging via caplog.
• The existing tests/test_cli.py was updated with an autouse fixture to stub out preflight during CLI tests, and the prompt-file behavior test was updated to reflect the new file-reading logic.
• Ran uv run pytest -q to ensure all tests pass.

Considerations & Alternatives

• Could have separately checked for the Git binary presence, but unified the worktree check to handle both missing Git and non-repo cases.
• Chose to accumulate all preflight errors and display them together, rather than failing fast on the first error.
• File-reading logic is local to the prompt-handling block; alternative designs could externalize prompt-loading to a helper.