[Watchlists] --sector CLI flag — run pipeline across a full GICS sector universe

[DogInfantry]

Part of epic: #31
Depends on: #38 (WatchlistManager must exist first)

What

Add --sector and --sector all arguments to pipeline_v2.py so users can run the pipeline across a full GICS sector instead of specifying individual tickers. Preserves existing --tickers behaviour.

CLI usage

# Run pipeline across all Technology sector tickers
python pipeline_v2.py --sector technology

# Run across all 11 sectors (deduplicating cross-listed names)
python pipeline_v2.py --sector all

# Existing behaviour preserved
python pipeline_v2.py --tickers AAPL MSFT NVDA

Implementation

parser.add_argument('--sector', type=str, default=None,
    help='GICS sector name or "all" to run across full sector universe')

# In main:
if args.sector:
    wm = WatchlistManager()
    if args.sector == 'all':
        tickers = wm.get_all_tickers()
    else:
        tickers = wm.get_sector(args.sector)
        if not tickers:
            raise ValueError(f'Unknown sector: {args.sector}. Available: {wm.list_sectors()}')
elif args.tickers:
    tickers = args.tickers
else:
    tickers = WatchlistManager().get_sector('technology')  # sensible default

Acceptance Criteria

• --sector technology runs pipeline on all 8 Technology tickers
• --sector all runs on full deduplicated universe without crashing
• --sector nonexistent raises clear ValueError with list of valid sectors
• --tickers flag still works (no regression)
• Both flags cannot be used simultaneously (argparse mutual exclusion group)
• README documents --sector with 3 usage examples