Curate demo and product vision drafts into design docs

[codeforester]

Summary

• Fold durable base-demo draft guidance into the canonical demo docs without promoting stale script names or manifest schema.
• Add product-direction, workspace, doctor, observability, adoption-signal, and adapter-quality guidance to existing design docs.
• Clarify future workspace onboarding boundaries and doctor finding principles.
• Leave the untracked draft files untouched for manual removal.

Follow-up Issues Created

• Add read-only workspace check and doctor commands #393 Add read-only workspace check and doctor commands
• Add project port health declarations #394 Add project port health declarations
• Add project Git remote health diagnostics #395 Add project Git remote health diagnostics
• Design local command history and explanation surfaces #396 Design local command history and explanation surfaces
• Add clean macOS install validation checklist #397 Add clean macOS install validation checklist

Validation

• git diff --check
• searched tracked docs for stale exact draft terms: manifest.yaml, base-demo.sh, Claude API, LinkedIn

Closes #392