app-server: add host config layer

[aaronl-openai]

Overview

Adds an app-server host config layer that behaves like a config.toml-equivalent layer for app-server behavior, so a host can set config once and have subsequently created threads resolve it consistently through the normal loader.

Changes

• Add v2 config/host/set plus AppServerHost layer metadata, generated schema output, and app-server docs.
• Merge host config through the canonical config loader above user config and below project/session layers, including base-dir path resolution and config/read layer/origin reporting.
• Route app-server config consumers through the shared layer, including thread startup paths, imported external sessions, hooks/skills/proxy treatment, and external-agent plugin detection.

Verification

• Manually verified with real codex app-server: host config is visible in config/read, respects project precedence, and clears cleanly.
• Manually verified with real codex app-server: host config reaches thread/start, cold thread/resume, thread/fork, and resumed imported external sessions.

git stack

• 👉 1 app-server: add host config layer #23770
• ⏳ 2 app-server: reload fork config on Windows #23769