Document the advanced factory roadmap

[finna]

What changed

• Added a maturity map from the three-skill starter to a fuller software factory.
• Documented fresh-reviewer convergence, bounded fix rounds, and the loop-stuck escape hatch.
• Added a Slack control-plane blueprint for blocked issues, merge-ready approvals, daily digests, and watchdog alerts.
• Documented risk-tiered merging, Linear issue chains, preview and documentation gates, morning/status skills, a durable pipeline model, leased workers, and a post-merge learning loop.
• Marked versioned Slack spec approval as the next experiment rather than shipped public functionality.
• Added a recommended implementation order and kept the human policy boundary explicit.

Why

The public repo intentionally contains only the portable three-skill core. Readers also need a clear path to the advanced systems proven in the larger factory without being misled into thinking those integrations are already included.

User impact

People can start small, understand which layer solves which operational bottleneck, and reproduce Slack or automation features without turning Slack into a second source of truth or weakening the human gates accidentally.

Validation

• node scripts/validate.mjs
• git diff --check

Intentionally not included

• No Slack, webhook, worker-fleet, or auto-merge implementation code.
• No credentials, private infrastructure details, or Henry-specific names.
• No change to the starter's default rule that humans merge.