[adjenk]

Adds --json to git why log, per #2, so the reasoning timeline can be piped into other tools instead of only read as coloured text.

cmd_log now branches on args.json early in its existing filter loop (still honours --agent, --session, --spec, --with-reason), and reuses log_records() / first_of_each() unchanged — no new parsing. Each matching commit is printed as one JSON object per line (NDJSON): commit (short hash), subject, date, plus a why/why_* key for each Why* trailer actually present on that commit, mirroring the field naming cmd_export already uses.

test/smoke.sh gained an assertion piping git why log --json through python3 -c "import sys,json; [json.loads(l) for l in sys.stdin if l.strip()]" to confirm it's valid NDJSON, plus a check that why_agent comes through.

• sh test/smoke.sh prints SMOKE OK
• still one file, no new dependencies
• committed with git why commit