Add official HarnessRouter skill

[kuanzema]

What changed

• adds the official harnessrouter Agent Skill
• adds progressive references for Agent Feature and Harness planning, Workspace and Harness configuration, runtime integration, Files/Artifacts/rendering, security, and end-to-end testing
• adds OpenAI-compatible Skill interface metadata
• adds the repository entry README

Why

Coding agents need a stable, purpose-built workflow for identifying agent-powered product features, configuring one or more HarnessRouter Harnesses, and connecting them securely to the host product without rebuilding a parallel agent layer.

Developer impact

Developers can load $harnessrouter from this repository and receive only the references required for the current integration task. The Skill requires a transcript-safe API-key modal at the start and preserves server-side authorization, tenancy, Streaming, Session, File, and Artifact boundaries.

Manual review gate

Richard Song must review this Skill against HarnessRouter's currently deployed and verifiable product capabilities before this Draft PR becomes ready or merges. Valid evidence is the deployed API schema, capability metadata, official product surfaces, and accepted runtime behavior. Roadmap items, internal candidates, and unaccepted Payments, Deployment, or Database capabilities must not be presented as currently available.

Validation

• quick_validate.py harnessrouter
• local Markdown link validation
• git diff --cached --check
• sensitive-pattern scan