refactor(core): remove orphaned message projector paths

[kitlangton]

What

Remove the test-only in-memory session message projector and event handlers that cannot run through the durable SQL projector.

How

• Remove SessionMessageUpdater.memory and its two adapter-specific tests.
• Narrow SessionMessageUpdater.update from all session events to SessionEvent.DurableEvent.
• Dispatch exhaustively over durable event types, removing unreachable ephemeral delta, progress, and usage handlers.
• Preserve all durable SQL projection handlers and behavior.

Scope

This does not change ephemeral event publication or live consumers. It only removes their unreachable paths from the durable message projector.

Testing

• bun typecheck from packages/core
• bun run test test/session-projector.test.ts test/session-runner.test.ts test/session-prompt.test.ts test/session-compaction.test.ts from packages/core (194 passed)
• git diff --check