Add execution principal context

[chubes4]

Summary

• Add AgentsAPI\AI\AgentExecutionPrincipal as an immutable runtime context for acting user, effective agent ID/slug, auth source, request context, optional token ID, and request metadata.
• Add user-session and agent-token construction helpers plus the agents_api_execution_principal resolver filter for host plugins.
• Expose and document the primitive while keeping access policy, scoped grants, token storage, and conversation-loop behavior out of scope.

Source

• Fixes Add generic agent execution principal #12

Tests

• composer test - passed

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted the execution principal primitive, resolver hook, smoke tests, and docs; Chris remains responsible for review and validation.