refactor(storage, conversation): Store base config separately

[JeanMertz]

Previously, events.json packed the conversation's base config as a synthetic first element — a ConfigDelta containing the full AppConfig. This made the file difficult to inspect: opening it in an editor meant scrolling past hundreds of lines of config JSON before reaching actual conversation events.

The base config is now stored in a separate base_config.json file, written once at conversation creation time and never overwritten. events.json contains only InternalEvent entries (config deltas and conversation events), with no synthetic leading element.

The custom Serialize/Deserialize impls on ConversationStream are removed in favour of explicit from_parts/to_parts methods used by the storage layer. InternalEvent is demoted from pub to pub(crate) since nothing outside jp_conversation references it.

The compat module's deserialize_config_delta is renamed to deserialize_partial_config and now operates directly on PartialAppConfig. A new deserialize_config_delta in stream.rs delegates to it for the timestamp + delta extraction pattern used when deserializing individual ConfigDelta stream events.

Backward compatibility is handled transparently in the load path: if base_config.json is absent, the loader falls back to extracting the base config from the first element of events.json. On the next persist, base_config.json is written and events.json is rewritten without the leading config entry.

Implements RFD 054.