Add Codex research skills and setup docs

Include Codex setup docs, installer updates, and switch the OpenCode web research agent default model to gpt-5.4.

skills/research-codex-en/research-add-items/SKILL.md

@@ -0,0 +1,28 @@
+---
+name: research-add-items
+description: Add items (research objects) to existing research outline.
+---
+
+# Research Add Items - Supplement Research Objects
+
+## Trigger
+`/research-add-items`
+
+## Workflow
+
+### Step 1: Auto-locate Outline
+Find `*/outline.yaml` file in current working directory, auto-read.
+
+### Step 2: Get Supplement Sources in Parallel
+Simultaneously:
+- **A. Ask user**: What items to supplement? Any specific names?
+- **B. Ask if Web Search needed**: Launch agent to search for more items?
+
+### Step 3: Merge and Update
+- Append new items to outline.yaml
+- Display to user for confirmation
+- Avoid duplicates
+- Save updated outline
+
+## Output
+Updated `{topic}/outline.yaml` file (in-place modification)