Add address book / contacts for named SS58 accounts

Add an address book feature that lets users store and reference SS58 accounts by name.

Motivation

Users frequently interact with the same accounts. Instead of copying addresses around, they should be able to assign human-readable names and use those names anywhere an address is expected — in dot tx, queries, etc.

This also enables AI agents to work with natural language like:

"send Paul 1 DOT on People chain from my faucet account"

Ideas

• dot contacts add <name> <ss58-address> — save a named contact
• dot contacts list — show all saved contacts
• dot contacts remove <name> — remove a contact
• Contacts should be usable wherever an SS58 address is expected (tx calls, storage queries, etc.)
• Consider where to persist contacts (e.g. ~/.config/polkadot-cli/contacts.json)
• Consider how this interacts with the existing account/keyring system — named accounts from the keyring should also be resolvable by name

Open questions

• Naming: contacts, addressbook, addresses, known?
• Should contacts be chain-specific or global (SS58 is chain-agnostic with prefix re-encoding)?
• Should we support importing contacts from common sources (e.g. a JSON file, polkadot.js address book)?

Start by reviewing the existing CLI command structure and account/keyring handling. The issue leaves the command naming, persistence model, chain scope, imports, and address-resolution behavior open. Done would require an agreed design plus contacts management and named-account resolution wherever an SS58 address is accepted.