Extract Runtime Kernel read model boundary

[likun666661]

Summary

• Introduce RuntimeEventStore, RuntimeReadModel, and RuntimeKernel as first-class runtime boundaries.
• Make RuntimeEvent the semantic read/model-history source for messages, turns, retry/regenerate/branch, recovery, and next-turn context.
• Demote AgentRunStore to run headers and operational lifecycle events; remove the legacy SessionStore semantic read fallback / env rollback path.
• Preserve public SessionEvent/UI/gateway contracts while making cache-only StoredMessage rows unable to drive semantic reads.
• Clone parent RuntimeEvents into branch child sessions so branch reads and model context remain RuntimeEvent-backed.

Verification

• npm run typecheck --workspace packages/runtime
• npm run build --workspace packages/runtime
• node --test packages/runtime/dist/__tests__/session-manager.test.js
• git diff --check

A full workspace npm run test --workspaces --if-present passed before rebasing this same commit onto the now-merged PR #14 base; the rebase was clean and the focused runtime checks above passed after rebase.