Agent config read/write module (JSON, JSONC, YAML) #432

Part of #430.

Task

Implement config read/write module for agent configuration files.

Supported Formats

1. JSON — serde_json with preserve-order + comments support
2. JSONC (JSON with comments) — strip // and /* */ before parse
3. YAML — serde_yaml

API

mod agent_config {
    fn read_config(path: &Path, format: Format) -> Result<AgentConfig>;
    fn write_config(path: &Path, format: Format, config: &AgentConfig) -> Result<()>;
    fn has_cora_entry(config: &AgentConfig) -> bool;
    fn add_cora_entry(config: &mut AgentConfig, binary_path: &str) -> Result<()>;
    fn remove_cora_entry(config: &mut AgentConfig) -> Result<()>;
}

Edge Cases

• File has BOM → strip before parse
• File has trailing comma in JSON → tolerant parse
• File permissions → return clear error
• File does not exist → create with just Cora entry
• Existing mcpServers key → merge into it
• No mcpServers key → create it

Acceptance

• Round-trip: read → add entry → write → read → verify entry present
• No data loss: existing MCP servers preserved
• JSONC comments preserved (if using serde_json with preserve)

Est: 2-3 days