docs: add AW harness design plan

Design plan for the 'aw' engine harness — a Pi SDK-based replacement
for copilot_harness.cjs that adds multi-step DAG orchestration,
multi-agent/multi-model support, context engineering, cost tracking,
steering, session repair, and observability.

Key design decisions:
- Built on Pi agent ecosystem (pi-coding-agent, pi-agent-core, pi-ai)
- All gh-aw capabilities implemented as Pi extensions (ExtensionAPI)
- Model resolution via existing api-proxy (no provider inference)
- TypeScript compiled for Node 24, bundled as single .cjs
- New opt-in engine (engine: aw) — existing engines untouched

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: pelikhan