Define package visibility for index.prose.md and sibling Contracts

[irl-dan]

Problem

OpenProse documents directory packages with an index.prose.md root and private sibling sub-units, but the language and resolver do not define package exports or enforce sibling privacy. Today, private is an authoring convention rather than a Contract boundary.

This surfaced while splitting std/ops/compose into an obligation-centered package. Only compose/index.prose.md should be publicly addressable; architecture, dialogue, source, view, and feedback Contracts are implementation details.

Evidence

• packages/std/ops/prose-author.prose.md recommends a multi-unit folder with index.prose.md plus private sub-units and validates a private sub-unit directory.
• skills/open-prose/deps.md defines directory-root resolution to index.prose.md, but does not define exports or visibility.
• Contract Markdown and Forme resolve Contracts by name/path without a package-private rule.

Desired property

A directory package has one self-evident public surface. Importing or running the directory resolves its index.prose.md; sibling Contracts remain package-private unless deliberately exported. Compile/lint should reject cross-package references to private siblings.

Smallest plausible change

Define index.prose.md as the sole public entrypoint by default and sibling *.prose.md Contracts as package-private. Add an explicit export mechanism only when a package needs multiple public Contracts. Document local sibling resolution separately from cross-package resolution and add compiler/lint coverage.

Alternatives

• Use directory placement such as private/ as the visibility boundary.
• Add frontmatter visibility to every Contract.
• Keep convention-only privacy, but stop claiming it is enforced.

This is framework feedback discovered during Compose work; implementation should happen separately from that design session.