Add timeout parameter to Agent contract

[jkudish]

The Promptable trait's prompt() and stream() methods accept ?int $timeout = null, but the Agent contract interface does not declare this parameter. This causes static analyzers like PHPStan to flag timeout: as an unknown argument when calling prompt() on the Agent return type — e.g. from the agent() helper.

// agent() returns Agent (the contract), which lacks $timeout
agent(instructions: '...')->prompt(
    prompt: '...',
    provider: 'claude-cli',
    model: 'claude-sonnet-4-5',
    timeout: 600, // PHPStan: Unknown parameter $timeout
);

This was introduced in #20 which added $timeout to the trait but didn't update the contract.

This PR adds ?int $timeout = null to both prompt() and stream() on the Agent interface to match the Promptable trait implementation.

[pushpak1300]

Added in #412

[pushpak1300]

Thanks.