Add 'revoke' and 'approve' subcommands to manage ERC-20 allowances from the CLI #31

Problem

The awal CLI lets agents trade tokens but provides no way to manage the standing ERC-20 allowances those trades create. Every swap from an ERC-20 requires approve(router, amount), and the SDK appears to default to max-uint256 approvals. A one-time swap leaves a permanent unlimited allowance.

If the spender contract is later compromised — or the user wants to scope down the agent wallet's exposure after a session — there is currently no way to revoke the allowance from inside the agent runtime. Workarounds (revoke.cash, etc.) require the user to control the same address through a separate wallet client, which breaks the agentic model.

Proposal

Add two subcommands:

npx awal@x.y.z approve <token> <spender> <amount> [--chain base|polygon]
npx awal@x.y.z revoke <token> <spender> [--chain base|polygon]

revoke is sugar for approve <token> <spender> 0. Same validation rules as trade/send. Output should match the trade JSON shape (transactionHash + basescan link).

Why this belongs in the CLI

Agentic wallets that can trade but cannot revoke are strictly worse than human-operated wallets from a security standpoint. Closing this gap is a prerequisite for trusting agents with non-trivial balances or longer sessions.

Adjacent

A companion skill (approvals-management, PR #30) covers the list side by querying Approval events. Pairing that with revoke would give agents a complete approval lifecycle: list → audit → revoke.

Tested scenario that motivated this

Agent wallet performed trade 2.5 USDC → AERO then trade 4.9 AERO → USDC on Base. Two standing approvals remain (USDC→router and AERO→router). With current tooling, the agent cannot revoke either.