refactor(core): centralize session message rows - #41830

What

Centralizes the persisted SessionMessage representation boundary in SessionMessageRow instead of repeating { id, type, data } assembly and splitting throughout session persistence.

How

• Adds packages/core/src/session/message-row.ts with effectful decode, synchronous decode, and synchronous encode operations.
• Migrates message persistence in the store, history, pending promotion, revert planning, projector, and transfer import paths.
• Keeps transaction handling, sequence numbers, timestamps, session ownership, and decode error policy at each call site.
• Keeps session_pending row codecs and fork row copying separate because those operations do not encode or decode the canonical message-row representation.
• Adds focused tests for representation round trips and canonical column precedence over stale JSON keys.

Scope

This is representation-only refactoring. It does not change the session_message schema, durable event behavior, pending input encoding, fork batching, or caller-specific error handling.

Testing

• bun run test test/session-message-row.test.ts test/session-projector.test.ts test/session-create.test.ts test/session-prompt.test.ts from packages/core: 86 passed
• bun typecheck from packages/core
• Push hook: typechecked all 39 workspace packages
• Prettier check on all changed files
• git diff --check