Add JSON output for market data commands

[xyjk0511]

Summary

• add --json output for market data commands so script users can pipe raw data
• support the issue example young a --zt --json without changing the existing table output
• document JSON pipeline examples and add parseability tests

Closes #2

Validation

• uv run --python 3.12 --with pytest --with click --with rich --with requests python -m pytest -q -> 18 passed
• uv run --python 3.12 --with ruff ruff check src tests -> passed
• python -m compileall src tests -> passed
• git diff --check -> passed

Notes

The JSON path stays in the CLI layer and reuses the existing structured core data so the current rich table output remains unchanged.

[AdvancingTitans]

Thanks for the JSON-output PR. I am not merging it as-is because GitHub marks it dirty/conflicting against current main, and the CLI output layer has changed in the 0.1.5 work: hk/us/stock now support --no-news, commands print session-stage labels, fund-flow has multi-source fallback semantics, and single-stock lookup needs to be included in any JSON story.

Please rebase on the latest main and keep the JSON path as a render-layer option without weakening quote/news/fund-flow source selection. Acceptance I would like to see before merge: young indices --json, young flow --json, young stock 0700.HK --json, young hk --no-news --json, and young us --no-news --json all produce valid JSON and preserve the same fallback behavior as text mode. Once rebased, this is a useful feature and worth revisiting.