Version and dispatch durable Fold events

[K-Mistele]

Summary

• Write version on every durable Fold event from the central stored-entry factory.
• Export the current and supported event-format versions for host capability negotiation.
• Decode persisted events through a version-dispatching Fold boundary and return a typed error for unsupported future versions.
• Preserve compatibility with historical entries that predate per-entry versioning by upcasting them to v1.
• Route the JSONL backend through the same decoder that Postgres-backed hosts such as Riptide should call.

Runtime model

• Hosts persist and transport the Fold event envelope without interpreting its version.
• The installed Fold runtime selects a bundled historical decoder for each entry and upcasts it to the current in-memory LogEntry model.
• Writers always emit CURRENT_LOG_ENTRY_VERSION; readers advertise SUPPORTED_LOG_ENTRY_VERSIONS.
• A future v2 keeps LogEntryV1 frozen, adds LogEntryV2 plus an upcaster, and extends decodeStoredLogEntry.
• Unknown versions fail with EventLogUnsupportedVersionError instead of being guessed or treated as transient storage failures.

Validation

• bun run typecheck
• bun run lint
• bun run format:check
• bun x vitest run test/EventLog/StoredLogEntryDecoder.vi.test.ts test/EventLog/EventLogMemory.vi.test.ts in packages/fold-core
• bun x vitest run test/EventLog/EventLogJsonl.vi.test.ts test/Session/TitleGenerator.vi.test.ts in packages/fold-agent
• bun x vitest run test/PromptRenderer.vi.test.ts test/Renderer.vi.test.ts test/TuiSubagents.vi.test.ts in packages/fold-cli