[erSpell]

Summary

• add a --json CLI flag that prints a single machine-readable object with answer and sources
• preserve the existing human-readable output when --json is not provided
• add pytest coverage for JSON output and document the flag in the README

Test plan

• uv run --extra dev python -m pytest -q
• uv run --extra dev python -m notes_rag --mock --json 'What is a git remote?' | python -m json.tool
• uv run --extra dev python -m notes_rag --mock 'What is a git remote?'

Closes #1