fix: restore Codex compatibility in WebUI chat

[Titan-Frank]

Summary

• update the headless Codex invocation for current CLI releases and translate the current thread/item JSONL event format
• surface non-zero Codex subprocess failures in the WebUI instead of silently completing with no reply
• bridge the legacy SSE DataFlow MCP endpoint through project-local mcp-proxy, safely migrating matching legacy Codex URL entries
• add installer regression checks and update setup/release documentation

Root cause

Codex CLI 0.149.1 rejects --approve-for-me combined with an explicit --sandbox value. Its JSON output also uses thread.started and item.* events, while the adapter only recognized the older msg.type shape. The process exited or emitted an unrecognized response, so the chat panel showed the user message without an assistant reply.

Verification

• 72 backend tests passed with an isolated DataFlow core directory
• make check passed (shell syntax, skills, generated assets, MCP whitelist, profiles, docs, Codex config checks)
• temporary-HOME Codex configuration install and TOML parse passed
• live WebSocket chat returned a Chinese greeting through the WebUI endpoint
• live Codex MCP call emitted normalized tool start/end events and returned 14 operator categories
• uv pip check passed for 251 packages