docs: add BitGoWasm API design conventions

[lcovar]

documents 7 architectural patterns enforced in code reviews across wasm-utxo, wasm-solana, and wasm-dot.

these are design decisions that could reasonably be made differently but we've standardized on:

1. uint8array everywhere - no hex/base64 on transaction api
2. bigint for amounts - never number or string for monetary values
3. enums not magic strings - type safety for transaction types, instruction types
4. return transaction objects from builders - not raw bytes
5. parsing separate from transaction - standalone parseTransaction() function, not .parse() method
6. follow wasm-utxo conventions - fromBytes, toBytes, get wasm(), getId
7. typed wrappers - keypair/pubkey classes, not string-encoded keys

distilled from otto's recurring review comments. prevents review churn by making the patterns explicit.

ref: wasm-dot PR #145 discussion about parse convention alignment