Make Chat Buddy iOS the primary app and add CI

[Luckycat133]

What changed

• marks Chat Buddy iOS as the primary active version in both READMEs
• adds an MVP → internal beta → TestFlight → App Store roadmap
• documents opportunistic BackgroundTasks behavior and Keychain-backed API key storage
• adds a macOS 26 / Xcode 26.5 GitHub Actions workflow
• fixes compiler errors exposed by the first clean CI build, including chat navigation, RAG indexing, model selection, Keychain logging, API client sendability, Markdown indexing, and data import

Why

The repository already contains Web migration support, Keychain storage, unit tests, and UI tests, but its public status and release path were unclear and pull requests had no automated Xcode validation.

Validation

• git diff --check
• iOS CI #19 passed on macOS 26 / Xcode 26.5
• app target compiled successfully
• Chat_Buddy_iOSTests passed on an iPhone 17 Pro iOS 26.5 simulator