docs/agents: reconcile --agent codex drift; decide codex(#207)/zcode(#218) support

[amondnet]

Summary

Two agent-related upstream additions — #207 Codex (ef36899) and #218 zcode (6f2db52) — are not in csp's agent set. Separately, csp's README advertises --agent codex but the Agent enum has no Codex variant — a doc↔impl drift.

csp status

• crates/csp/src/bin/csp/main.rs:37 enum Agent = { Antigravity, Claude, Commandcode, Copilot, Cursor, Gemini, Kiro, Opencode, Pi, Reasonix } — no Codex, no Zcode.
• README.md / README.ko.md public API lists --agent <claude|cursor|codex|opencode|copilot|kiro|gemini> → codex documented but unimplemented.
• Note: csp's agent set already diverges from upstream (it adds commandcode/pi/reasonix/antigravity that upstream lacks), so agent parity is a deliberate-divergence question, not a straight port.

Proposed work

• Fix the README drift: either implement a Codex agent variant, or remove codex from the documented --agent list (EN + KO in sync).
• Decide whether to add codex (#207) and/or zcode (#218) to csp's agent set.

Refs

• Upstream: feat: Add Codex subagent support MinishLab/semble#207 (ef36899), #218 (6f2db52)
• Sync baseline check: eacbe43..949559b, 2026-07-11