Add queryable history, hidden compaction, and runtime observation

[timazed]

Summary

• add query-first runtime persistence with GRDB-backed storage, paged history inspection, lightweight thread/context reads, and whole-thread deletion/redaction support
• externalize persisted image attachments into sidecar files so runtime history stays small and restore/paging performance does not degrade with image-heavy threads
• add hidden effective-context compaction that preserves canonical user-visible history while compacting only the runtime prompt working set
• add first-class runtime context-usage projection APIs (fetchThreadContextUsage, observeThreadContextUsage, and AgentThreadContextUsage) so hosts can inspect token pressure without rebuilding those estimates in app code
• improve messaging behavior so streamMessage(...) returns immediately, commits the local user message before backend startup finishes, and sendMessage(...) can be paired with live observation cleanly
• add Combine-based runtime observation APIs (observeThreads, observeThread, observeMessages, observeThreadSummary, observeThreadContextState, observeThreadContextUsage) plus setTitle(_:for:)
• update the demo app to exercise compaction, token-based context usage, observation, and thread renaming, and refresh the README/docs to match the new feature set

Testing

• swift build
• swift test
• xcodebuild -project DemoApp/AssistantRuntimeDemoApp.xcodeproj -scheme AssistantRuntimeDemoApp -sdk iphonesimulator -destination 'generic/platform=iOS Simulator' CODE_SIGNING_ALLOWED=NO build