feat: add broker-neutral derivative discovery

[felipecsl]

Context

Raw IBKR scripts were previously required to distinguish NQ futures options and alternate index trading classes. This implements issue #46 using a capability-specific boundary rather than widening the shared account-oriented broker client.

What changed

• defines durable derivative expiry, contract, quote, and market-data availability contracts
• adds an IBKR adapter that keeps conids as opaque broker references
• adds a memoized derivative capability resolver with clear unsupported-broker errors
• preserves exact asset class, trading class, exchange, multiplier, settlement, and exercise metadata
• covers real captured NQ QN3 and NDX/NDXP response shapes without invented provider fields
• consumes the published @huskly/ibkr-client@^0.8.0 capability directly

Safety and compatibility

Discovery exposes no placement, What-If, warning-reply, cancellation, or raw-endpoint surface. Ambiguous identity and unusable chains fail closed. Existing Schwab commands remain unchanged.

Validation

• yarn run check
• yarn build
• 7 test files passed

Closes #46