Migrate developer Markdown docs into DocC

[leogdion]

Summary

The repo now keeps its long-form developer documentation under Documentation/:

• Documentation/CONTEXT_ENGINE.md — ContextEngine anatomy and runtime behavior (with diagrams)
• Documentation/WATCH_COMMS_RELIABILITY.md — the watch↔phone reliability rationale and debugging trace
• Documentation/DEBUGGING.md — CLI recipes for live-debugging watch↔phone apps on device

These should eventually be migrated into the Swift-DocC catalog at Sources/SundialKitStream/SundialKitStream.docc/ so they are published as part of the rendered documentation (e.g. on Swift Package Index) and stay versioned alongside the API symbols.

Proposed work

• Convert each Markdown file into a DocC article (.md with DocC directives / proper @Metadata where useful).
• Wire the articles into the documentation catalog's table of contents (Documentation.md top-level curation).
• Convert cross-document relative links (./CONTEXT_ENGINE.md, etc.) into DocC symbol/article links (<doc:...>).
• Link relevant API symbols (ContextEngine, RevisionedMessage, ExpiringMessage, StaleWindow) to the articles via curation.
• Update README.md links to point at the published DocC pages once available.
• Remove the Documentation/ Markdown copies (or leave thin pointers) once migration is complete.

Notes

The DEBUGGING.md recipes are device/CLI oriented and may be better as a standalone "Debugging on device" article rather than curated under an API symbol.