feat: support thought_level config option for codex-acp

[dutifulbob]

Summary

ACP spec defines thought_level as a standard SessionConfigOptionCategory for controlling reasoning/thinking levels. However, codex-acp (v0.9.3) doesn't implement session/set_config_option yet — it returns -32602 Invalid params.

Goal

Make acpx codex set <configId> <value> work for controlling thinking/reasoning level when using codex.

Options

1. Upstream fix: contribute session/set_config_option support to codex-acp (@zed-industries/codex-acp)
2. Workaround: if codex-acp exposes thinking via CLI flags or env vars, pass them at agent spawn time instead of through ACP config
3. Graceful fallback: improve acpx error handling so set returns a clear message when the agent doesn't support config options (instead of [object Object])

Context

• ACP spec: SessionConfigOptionCategory includes thought_level
• codex-acp returns JSON-RPC -32602 for session/set_config_option
• session/set_mode also returns -32602 (same root cause — codex-acp doesn't implement these methods)
• acpx already has the client-side plumbing: acpx codex set <key> <value> and acpx codex set-mode <mode>

Related

• Discovered during manual testing of full ACP coverage (PR fix: cancel prompt during startup #10)
• JSON-RPC error wrapping bug (raw {code, message} objects instead of Error instances) should also be fixed