[Watchlists] config.yaml watchlists section — all 11 GICS sectors with S&P 500 tickers

[DogInfantry]

Part of epic: #31

What

Add a watchlists: section to config.yaml with all 11 GICS sectors pre-populated with the top 6-8 S&P 500 names per sector by market cap. This is the static data foundation that watchlist.py (#38) and the --sector CLI flag (#39) will read from.

Sectors to populate

Sector	Key names
technology	AAPL, MSFT, NVDA, META, GOOGL, AMD, AVGO, ORCL
healthcare	JNJ, UNH, LLY, ABBV, MRK, TMO, ABT, DHR
financials	JPM, BAC, WFC, GS, MS, BLK, C, AXP
energy	XOM, CVX, COP, SLB, EOG, PSX
industrials	CAT, HON, UNP, GE, RTX, LMT, DE
consumer_discretionary	AMZN, TSLA, HD, NKE, MCD, SBUX, LOW
consumer_staples	PG, KO, PEP, WMT, COST, PM
utilities	NEE, DUK, SO, AEP, EXC
materials	LIN, APD, ECL, NEM, FCX
real_estate	PLD, AMT, EQIX, SPG, DLR
communication_services	META, GOOGL, NFLX, DIS, CMCSA, VZ, T

Acceptance Criteria

• config.yaml has a top-level watchlists: key
• All 11 GICS sectors present as sub-keys
• Each sector has 5-8 tickers, all valid Yahoo Finance symbols
• Tickers are deduplicated (META and GOOGL appear in multiple sectors — flag this clearly in comments)
• A custom: sector slot exists for users to add their own tickers
• YAML file is valid (passes yaml.safe_load without error)