Create a Base product requirements document

[codeforester]

Base has several maintained product and technical documents, including README.md, docs/why-base.md, docs/architecture.md, docs/tool-boundaries.md, and docs/product-assessment.md. What is missing is a concise product requirements document that states the accepted product intent, user workflows, scope boundaries, requirements, and maintenance rules in one canonical place.

Create a PRD that:

• Defines Base as a macOS-first local workspace control plane for multi-repo developers.
• Separates product intent and requirements from implementation details.
• Links to existing technical and assessment docs instead of duplicating them.
• Captures target users, jobs to be done, scope, non-goals, success criteria, and accepted requirements.
• Documents how the PRD should stay current as Base evolves.
• Updates the documentation map and AI context index so contributors and agents know where the PRD fits.

The PRD should be created through a pull request and left for maintainer review before merge.