fix: Error on invalid JSON in framework settings instead of silently overwriting

Problem

When running mi6 enable, if a framework's settings file (e.g., ~/.claude/settings.json) contains invalid JSON, mi6 silently overwrites the entire file with generated config.

Current behavior:

let existing: Option<serde_json::Value> = if settings_path.exists() {
    let contents = std::fs::read_to_string(settings_path)?;
    serde_json::from_str(&contents).ok()  // ← Silent failure returns None
} else {
    None
};

When .ok() returns None, the merge logic treats it as "no existing config" and overwrites the file.

User impact:

• User manually edits settings, makes a typo (missing comma, etc.)
• Runs mi6 enable
• All their settings are gone, replaced with mi6's generated hooks
• No warning, no backup, no indication anything went wrong

Proposed Fix

Error out with a clear message instead of silently overwriting:

let existing = if path.exists() {
    let contents = fs::read_to_string(path)?;
    match serde_json::from_str(&contents) {
        Ok(v) => Some(v),
        Err(e) => {
            return Err(InitError::InvalidSettings {
                path: path.to_path_buf(),
                error: format!("Invalid JSON: {e}"),
            });
        }
    }
} else {
    None
};

Output:

error: ~/.claude/settings.json contains invalid JSON
  → line 15: expected ',' or '}'
  
Fix the JSON syntax and retry, or use --force to overwrite.

Scope

• Only affects the JSON parsing in install.rs merge logic
• Add --force flag to explicitly allow overwrite (opt-in destruction)
• Similar fix needed for TOML parsing (Codex)