Add Codex chat forking for trace debugging

[goyalayus]

Summary

This adds a Codex-only path for trace debugging where an existing Codex session can be used as context without mutating the original session.

When the message pane is attached to a trace and Codex is the active provider, selecting an existing Codex chat marks it as the fork source. The next message creates a new Codex session file from that source, attaches the current trace context, and continues through the existing codex exec --json streaming path. Claude behavior is unchanged.

Why

codex fork is currently a top-level interactive TUI command, so it cannot be used directly from Workshop's JSON streaming chat. This patch performs the fork at the Codex session-file layer, then resumes the new session through the non-interactive JSON path Workshop already supports.

Verification

• bun x eslint app/src/components/MessagePane.tsx src/codex-sessions.ts src/server.ts — passes with one existing React hooks warning in MessagePane.tsx.
• bun x tsc --noEmit --pretty false
• cd app && bun x vite build
• Headless Chrome UI smoke test against local Workshop: opened a trace, selected an existing Codex chat, saw the fork notice, sent a message with /api/agent/messages intercepted, and verified the browser sent session_id: null, a non-empty fork_session_id, the trace run_id, and the entered content.
• Session-file smoke test against a real local forked Codex session: verified the first session_meta is treated as the current session and ancestor metadata is preserved.

Notes

cd app && bun x tsc --noEmit --pretty false still fails on the existing React/lucide type-version mismatch across the app, plus existing unrelated errors. The root/backend typecheck passes.