[Feature]: delete session state file when session is deleted

[tomasWade]

DCP persists pruned state per session at ~/.local/share/opencode/storage/plugin/dcp/{sessionId}.json. When a session is deleted (via CLI or UI), the corresponding state file remains on disk indefinitely as an orphan. Over time these accumulate and waste space.

The opencode SDK already emits a session.deleted event with the session ID:

type EventSessionDeleted = {
    type: "session.deleted";
    properties: {
        info: Session;
    };
};

The existing event handler in lib/hooks.ts (createEventHandler) currently ignores all events except message.part.updated. Adding a session.deleted branch that unlinks the state file would fix this:

if (input.event.type === "session.deleted") {
    const sessionId = input.event.properties.info?.id
    if (sessionId) {
        const filePath = join(STORAGE_DIR, `${sessionId}.json`)
        await fs.unlink(filePath).catch(() => {})
    }
    return
}

Similarly, on plugin startup or when handling session.created, we could sweep orphaned state files for sessions that no longer exist in the DB.