good first issue: --json CLI output for reply + mode

[primeodin]

Why

First-timers often want to pipe the CLI into another script. A --json flag teaches structured output without growing the package.

Done when

• python -m first_commit_ai --mock --json "hi" prints one JSON object on stdout
• Fields include at least: reply (string), mock (bool), and system (string used)
• Non-JSON mode stays plain text (no behavior change for existing users)
• One new pytest covers the mock + --json path
• README 60-second start shows one --json example line

Out of scope

Streaming, multiple turns, or new HTTP clients.

Part of the PrimeOdin daily AI teaching builds.