fix: support mainnet CLI network and payment guidance

[davidgasquez]

Fixes #352

• propagate --mainnet and network selection through CLI auth and RPC resolution
• reuse a single helper for mainnet USDFC acquisition guidance in validation and payments status
• make config defaults tests independent from ambient NETWORK and RPC_URL

Validation:

npm test
# Test Files  35 passed (35)
# Tests  336 passed | 11 skipped (347)

[rvagg]

Suggested change

	/** Commander shorthand for --network mainnet */
	mainnet?: boolean | undefined

[rvagg]

I pushed another commit that removes a few things, commander's implies() does most of the validation work for us with --mainnet and --network so we shouldn't be double-validating. Passing on the chain is the main fix here, the USDFC additions are very nice, and the env cleanup is good too.