Add UI-independent reference for creating a Fabric account, organization, and cluster

### Gap

`fabric/create-account.md`, `fabric/create-organization.md`, and `fabric/cluster-creation-management.md` are **UI click-throughs** ("click the 'New Organization' button", "click the three-dots menu", screenshots) rather than atomic, action-oriented reference. UI-coupled content drifts with Studio layout changes and is poor source material for automation and agent rules. (The one reference-shaped section — "Connecting the Harper CLI to a Cluster" — is already solid.)

### Impact

The `@harperfast/skills` rule `creating-a-fabric-account-and-cluster` must stay hand-authored because there's no UI-independent canonical flow to source from. Minor factual drift also exists: the skills rule says "Config tab" while the docs say "Config → Overview".

### Suggested fix

Add an atomic, UI-independent "getting set up on Fabric" reference describing the account → organization → cluster sequence in terms of concepts and outcomes rather than current button labels.

_{Filed while auditing rule/doc coverage for @harperfast/skills. sent with Claude Opus 4.8}

Read fabric/create-account.md, fabric/create-organization.md, and fabric/cluster-creation-management.md, then compare them with the creating-a-fabric-account-and-cluster skills rule. Create an atomic reference covering the account → organization → cluster sequence by concepts and outcomes rather than UI labels. Done means the flow is UI-independent and can serve as canonical source material for the rule.

Written by the indexing model from the issue text.