[BUG] [v0.0.7] JsonFileStorage crashes on empty file (serde_json parse error)

[climax-dev-1]

Description

JsonFileStorage::new in src/cortex-engine/src/memory/store/json_file.rs will panic/return an error when the backing file exists but is empty. This happens because serde_json::from_str("") returns an error for an empty string, unlike the SqliteStorage backend which correctly handles the empty-file case.

Affected File

src/cortex-engine/src/memory/store/json_file.rs, lines 25-39

pub async fn new(path: PathBuf) -> Result<Self> {
    let memories = if path.exists() {
        let content = tokio::fs::read_to_string(&path).await?;
        let list: Vec<Memory> = serde_json::from_str(&content)?;  // <-- crashes if content is empty string
        list.into_iter().map(|m| (m.id, m)).collect()
    } else {
        HashMap::new()
    };
    ...
}

Root Cause

When a JSON memory file is created (e.g., by save() writing an empty []) and then the process is killed before writing, or when the file is created externally as empty, serde_json::from_str("") returns Err(EOF while parsing a value). This propagates as an error from JsonFileStorage::new, preventing the memory system from initializing.

Compare with SqliteStorage::new (lines 28-35) which correctly handles this:

let content = tokio::fs::read_to_string(&path).await.unwrap_or_default();
if content.is_empty() {
    HashMap::new()
} else {
    let list: Vec<Memory> = serde_json::from_str(&content).unwrap_or_default();
    ...
}

Steps to Reproduce

1. Create an empty file at the path used for JsonFileStorage
2. Call JsonFileStorage::new(path)
3. Observe error: EOF while parsing a value at line 1 column 0

Expected Behavior

An empty file should be treated as an empty memory store (no memories), matching the behavior of SqliteStorage.

Actual Behavior

JsonFileStorage::new returns an error, preventing the memory system from initializing.

Fix

let content = tokio::fs::read_to_string(&path).await?;
let list: Vec<Memory> = if content.trim().is_empty() {
    Vec::new()
} else {
    serde_json::from_str(&content)?
};