Build only prototypes selected for publication

[mdroidian]

Problem

CI determines which prototypes need to be packaged and published, but runs the root pnpm build first. That compiles every prototype even when only one prototype changed or needs publication.

Expected behavior

CI should compile only the prototypes selected for publication.

• Pull requests and normal main pushes should build directly changed prototypes only.
• A run with no selected prototypes should skip prototype compilation and artifact preparation.
• Manual full publication and the missing-comparison-SHA safety fallback should still build every prototype because every prototype will be published.
• Required shared dependencies should still be built when needed.

Implementation notes

• Move prototype selection before the build step.
• Pass the selected prototype slugs into the build command, for example through Turborepo filters or an equivalent explicit workspace selection.
• Use the same selection for compilation and artifact preparation so CI cannot publish a prototype that was not built in that run.

Acceptance criteria

• Changing one prototype compiles and prepares only that prototype and its required dependencies.
• Changing multiple prototypes compiles and prepares exactly those prototypes and their required dependencies.
• A change outside prototypes/** does not compile or prepare prototypes automatically.
• Manual full publication compiles and prepares all prototypes.
• The comparison-SHA fallback compiles and prepares all prototypes.
• Tests verify the selection is applied consistently to both build and artifact preparation.

Context

Follow-up to #13. Publication concurrency and source ordering are tracked separately in #16.