Add stock tracking agent sample

[sevenYY]

Summary

• Add a standalone Python stock tracking agent sample for US, HK, and A-share markets
• Add configurable watchlist rules for PE, dividend yield, manual buy prices, and safety margin fallback
• Add a standard-library Web UI with filters, source switching, table display, recommendation cards, and JSON API
• Default the Web UI to live quotes on every request and disable browser caching so stale offline fixtures are not shown as current data
• Use Nasdaq quote API for latest US prices with Yahoo intraday chart fallback while keeping Yahoo fundamentals
• Replace A-share live data source with EastMoney and compute A-share dividend yield from trailing-12-month cash dividends
• Update the offline sample quote for AAPL to USD 312.06 and 贵州茅台 to CNY 1326 with TTM dividend yield 3.92%

Testing

• python3 -m unittest samples.stock_agent.test_stock_agent
• python3 samples/stock_agent/stock_agent.py --offline --market US --top 5
• Restarted python3 samples/stock_agent/web_agent.py --host 127.0.0.1 --port 8000
• Verified default Web UI sends Cache-Control: no-store, max-age=0
• Verified http://127.0.0.1:8000/?source=offline&market=US&top=5 returns 200 and includes AAPL at 312.06
• Verified http://127.0.0.1:8000/api/report?source=offline&market=US&top=5 returns 200 JSON with AAPL price 312.06
• Verified default live API fails explicitly in this cloud environment instead of falling back to stale offline data

Notes

• Live external quote requests from this cloud environment still return SSL EOF network errors for Nasdaq, Yahoo, and EastMoney; offline mode remains available for reliable local demos.