fix(sales-assist): resolve AI Assist agent from params, company setting, then fallback

[FredPeal]

Summary

The AI Assist channel created after a SalesAssist lead pull (and now also from the SalesAssist Create Contact Channel workflow activity) resolves its agent with this precedence:

1. ai_assist_agent_id workflow param
2. the company's ai_assist_agent setting (new ConfigurationEnum::AI_ASSIST_AGENT_ID)
3. the agent_id the step already runs with

The enabled check plus this resolution now live in one place, CreateAIAssistChannelAction::ifEnabled(), which returns null when AI Assist is off so callers just ?->execute().

Fixes along the way

• CreateSocialChannelsAfterPullAction read an undefined $params (should be $this->params), so the param override never applied.
• CreateSocialChannelActivity read $this->agentId, which does not exist on the activity. With no company setting this passed null to an int and threw a TypeError.
• The company setting value is cast to int before reaching the action.
• The hardcoded 'ai_assist_agent' key moved into Kanvas\Intelligence\Enums\ConfigurationEnum.

Tests

• CreateAIAssistChannelActionTest::testIfEnabledReturnsNullWhenAiAssistIsOff
• CreateAIAssistChannelActionTest::testIfEnabledResolvesAgentByPrecedence
• Existing PullLeadActivityReynoldsResolvedLeadTest covers the activity path.

PHPStan (level 0) clean on src/Domains/Connectors/SalesAssist and src/Domains/Intelligence/Enums. php-cs-fixer clean on every touched file.