Document the built-in agents

Improvements

• CoreRevert change that caused headers to be double merged if provider was authenticated in multiple places
• CorePrevent double-prefixing of Bedrock cross-region inference models@sergical
• CorePrioritize OPENCODE_CONFIG_DIR for AGENTS.md@lgladysz
• TerminalRestore direct OSC52 support
• DesktopTighten up session padding-top for mobile@DNGriffin