Add design spec: hook-integrated MCP engine

[5uck1ess]

Summary

Design spec for redesigning the devkit engine from a subprocess-spawning CLI to an MCP server + PreToolUse hook that enforces deterministic workflow execution inside Claude Code.

Based on research from 5 parallel agents covering: plugin architecture, hook mechanics, deterministic enforcement patterns, token efficiency, and LLM orchestration.

Core problem solved

The current engine spawns claude -p as a subprocess — which doesn't work inside Claude Code (OAuth token issue). This spec replaces that with an MCP server that controls what tools Claude can see per workflow step.

Key decisions

• MCP tool scoping as primary enforcement (Claude can't call tools that don't exist)
• PreToolUse exit 2 as safety net (hard blocks out-of-step actions)
• Condensed principles (~120 tokens) injected per step instead of full skill files (~800 tokens)
• ~65% token reduction (17k vs 50k+ for 8-step workflows)
• bin/ auto-PATH for binary distribution (no install script needed in Claude Code)
• $CLAUDE_PLUGIN_DATA for state (survives plugin updates)

Spec: docs/superpowers/specs/2026-04-10-hook-integrated-mcp-engine-design.md

Test plan

• Review spec for completeness and feasibility
• Approve before implementation planning begins