feat(cli): add --json output for reply and mode (#5)

[AhmadBilalDSA]

Closes #5

[AhmadBilalDSA]

Summary

Adds an optional `--json` CLI flag allowing callers to pipe structured output without external parsing dependencies.

Changes

• `src/first_commit_ai/cli.py`: Added `--json` argument to `build_parser()`. When enabled, outputs a JSON object containing `reply`, `mock`, and `system` fields. Preserves standard plain text output when omitted.
• `tests/test_cli.py`: Added test validating JSON payload structure, required keys, and types using `capsys` under `--mock` and `--json`.
• `README.md`: Documented `--json` usage in the 60-second start section.

Verification

• Ran `pytest` (8/8 passed).
• Manual check: `python -m first_commit_ai --mock --json "hi"` prints a valid single-line JSON string."