docs: update README for Pool 1.0.16

[amypoolside]

Summary

• Document Pool 1.0.16 features, including mid-turn steering, Auto approval mode, Hooks, and subagents
• Add update notifications and new session-management commands
• Clarify Plan-to-Build review, agent-dependent behavior, settings scope, and permission merging
• Advertise mid-turn steering in the ACP feature list
• Link detailed configuration and safety guidance to docs.poolside.ai

Validation

• git diff --check