fix+feat: unbreak CI + Arbitrum AgentAccount primitives

[abhicris]

CI root cause

cargo clippy -- -D warnings has been red for 16 days due to 7 accumulated lint errors:

• 3 unused imports (SwapParams in dex/router.rs, Address in dex/types.rs, U256 in oracle/mod.rs)
• chain field on UniswapV3Router never read
• Manual impl Default for AgentBuilder when #[derive(Default)] suffices
• type_complexity on the alloy filled-provider in ChainConnector
• WalletManager::next shadowing Iterator::next

Fixes

• Drop stale imports.
• Expose UniswapV3Router::chain() (now the field is read, also useful surface).
• #[derive(Default)] for AgentBuilder.
• Alias the alloy provider as DefaultFilledProvider; add ChainConnector::provider() so downstream modules can call contracts against it.
• Rename WalletManager::next -> next_wallet.
• cargo fmt normalized a few pre-existing formatting drifts.

Feature: Arbitrum AgentAccount primitives

Why this matters (agent-economy thesis)

arka is the Rust SDK layer that sits under agent-registry interactions. The first agent-economy deployments target Arbitrum with USDC settlement — that substrate needs first-class support in the SDK.

An agent's lifecycle on chain looks like: register -> deposit USDC -> earn fees -> execute paid tasks -> withdraw. This PR turns that lifecycle into a chain-agnostic trait (AgentAccount) with a working Arbitrum client as the first concrete implementation. Future registries (Base, Optimism, Tempo) plug in behind the same trait without the agent logic needing to know.

What's new

• src/agent/account.rs — AgentAccount trait: deposit, balance, withdraw, execute_task. Shape matches AgentDeposit-style registries, ERC-4337 smart accounts, and x402 escrows. Ships with InMemoryAgentAccount, a working (not stubbed) registry for tests and local simulation. TaskReceipt carries task id, agent, fee, and success.
• src/chains/arbitrum.rs — ArbitrumContracts static addresses (native USDC, USDC.e, USDT, WETH, Uniswap V3 router + quoter) + AgentDepositClient. The client builds calldata for an IAgentAccount Solidity interface via alloy::sol! — deposit(uint256), balanceOf(address), withdraw(uint256), executeTask(bytes32,bytes). Calldata-only by design: callers feed it into their signing/broadcast pipeline, so the client is testable without an RPC.
• examples/agent_account.rs — runs the in-memory flow end to end and prints real Arbitrum deposit calldata ready for submission.
• Prelude — AgentAccount, InMemoryAgentAccount, TaskReceipt, AgentDepositClient, ArbitrumContracts are now in arka::prelude::*.

Tests

14 unit tests, all green locally:

• chains::arbitrum::* — exact selector bytes (0xb6b55f25 deposit, 0x70a08231 balanceOf), calldata lengths, USDC wiring, chain-id alignment, custom settlement-token override, executeTask layout.
• agent::account::* — deposit/balance roundtrip, withdraw math, overdraw rejection, fee-charging + receipt emission, registered-agent counting.

Test plan

• cargo check --all-features (clean)
• cargo clippy --all-targets -- -D warnings (clean)
• cargo fmt -- --check (clean)
• cargo test --lib — 14/14 pass
• CI green on push

-- kcolbchain / Abhishek Krishna