Fixes issue #233 and #220.
## Changes
### 1. Fix /mode command not taking effect (Issue #233)
When switching mode via `/mode yolo`, the new mode was saved to the agent
but the existing session continued using the old mode. This caused YOLO
mode to not work until a new session was manually started.
Fix: Reset the session when mode changes, similar to how /model works.
File: core/engine.go
### 2. Fix --json flag position in codex exec resume (Issue #220)
The --json flag was placed before the thread_id positional argument in
`codex exec resume` command, causing "unexpected argument '--json'" error
on the second message in a session.
Fix: Move --json to the correct position after thread_id and before prompt.
Before: codex exec resume --json --skip-git-repo-check <tid> <prompt>
After: codex exec resume --skip-git-repo-check <tid> --json --cd <dir> <prompt>
Files: agent/codex/session.go, agent/codex/session_test.go
Co-authored-by: Ubuntu <ubuntu@localhost.localdomain>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>