Add Network migration CI baseline

[rchatham]

Summary

Starts the follow-up migration PR series by adding CI coverage, aligning deployment baselines with Network framework availability, and documenting the planned PR automation loop.

Changes

• Add GitHub Actions CI for Swift Package tests and Xcode project tests.
• Restrict workflow permissions to read-only repository contents.
• Raise declared deployment baselines to iOS 13 and macOS 10.15 in SwiftPM and CocoaPods metadata.
• Align Xcode project/test deployment targets with the iOS 13 baseline.
• Make the placeholder connection-manager test use an explicit MCPeerID-safe display name so CI host names do not affect test results.
• Add NetworkMigrationPRPlan.md with guardrails, subagent automation loop, PR sequencing, SwiftUI browser replacement plan, and security gates.
• Update README badges and link the Network migration docs.

Verification

• swift test
• xcodebuild test -project PeerConnectivity.xcodeproj -scheme PeerConnectivity -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.3.1' -configuration Debug

Notes

• This is intended as a stacked follow-up on PR feat: introduce Network framework transport prototype #22 (feature/network-framework-migration).
• No runtime behavior changes are included.

[rchatham]

Superseded by consolidated PR #29. No commits were discarded; this branch is preserved as a recovery and reference point. Addressed review findings were replied to and resolved before consolidation.