docs: Add missing commands, features, and settings across 6 guides

Phase 2 of docs sync: fill gaps in existing content for Jan-Mar 2026 features.

- Add 15 missing slash commands to built-in commands table
- Add bundled skills section (/simplify, /batch, /debug, /loop, /claude-api)
- Add ${CLAUDE_SKILL_DIR} string substitution for skills
- Add MCP OAuth metadata override, scope terminology update, CLAUDEAI toggle
- Add InstructionsLoaded/Setup hook events, HTTP hooks version note, new fields
- Add effort level visual indicators and autoMemoryDirectory/claudeMdExcludes settings
- Update /context command description with optimization suggestions
- Add update-plan.md tracking document

resources.md

@@ -16,6 +16,12 @@
 | Anthropic Cookbook | Code examples and tutorials | [github.com/anthropics/anthropic-cookbook](https://github.com/anthropics/anthropic-cookbook) |
 | Claude Code Skills | Community skills repository | [github.com/anthropics/skills](https://github.com/anthropics/skills) |
 
+## Anthropic Engineering Blog
+
+| Article | Description | Link |
+|---------|-------------|------|
+| Code Execution with MCP | How to solve MCP context bloat using code execution — 98.7% token reduction | [anthropic.com/engineering/code-execution-with-mcp](https://www.anthropic.com/engineering/code-execution-with-mcp) |
+
 ---
 
 ## Mastering Claude Code in 30 Minutes