Add canonical root quickstart for posts-authors-tags

[tonyketcham]

Source: flatbread-flow-pmf-audit.md

Project metadata

• Phase: Docs/onboarding quickstart
• Sequence: 4
• Priority: P0
• Suggested owner role: docs
• Supporting roles: DX
• Dependency status: blocked
• Blocked by: Publish focused positioning statement and non-goals #142 Publish focused positioning statement and non-goals; Define Flatbread relational primitives glossary #143 Define Flatbread relational primitives glossary

Task details

• Create the canonical first-success path using a posts/authors/tags relation model.
• Guide a new developer from install through a typed query result without requiring early GraphQL knowledge.

Acceptance criteria

• Quickstart starts from the repository root or a clearly named package/example path.
• Commands are canonical and match the current repo layout.
• The flow ends with a typed query result for posts, authors, and tags.
• GraphQL is introduced later as an optional interface, not as the first concept.