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

03-skills/README.md

@@ -217,6 +217,7 @@ Skills support dynamic values:
 | `$ARGUMENTS` | All arguments passed when invoking the skill |
 | `$ARGUMENTS[N]` or `$N` | Access specific argument by index (0-based) |
 | `${CLAUDE_SESSION_ID}` | Current session ID |
+| `${CLAUDE_SKILL_DIR}` | Directory containing the skill's SKILL.md file |
 
 **Example:**
 
@@ -725,6 +726,20 @@ Skill descriptions are loaded at **2% of the context window** (fallback: **16,00
 | **MCP** | Real-time | External data/service access |
 | **Hooks** | Event-driven | Automated side effects |
 
+## Bundled Skills
+
+Claude Code ships with several built-in skills that are always available without installation:
+
+| Skill | Description |
+|-------|-------------|
+| `/simplify` | Review changed files for reuse, quality, and efficiency; spawns 3 parallel review agents |
+| `/batch <instruction>` | Orchestrate large-scale parallel changes across codebase using git worktrees |
+| `/debug [description]` | Troubleshoot current session by reading debug log |
+| `/loop [interval] <prompt>` | Run prompt repeatedly on interval (e.g., `/loop 5m check the deploy`) |
+| `/claude-api` | Load Claude API/SDK reference; auto-activates on `anthropic`/`@anthropic-ai/sdk` imports |
+
+These skills are available out-of-the-box and do not need to be installed or configured. They follow the same SKILL.md format as custom skills.
+
 ## Sharing Skills
 
 ### Project Skills (Team Sharing)