feat: JSON streaming and JsonLineEvent

[joshrotenberg]

Summary

Parse codex's --json NDJSON output for structured event handling.

• CodexWrapper.JsonLineEvent struct: event_type, data (map), raw (string)
• CodexWrapper.Exec.execute_json/2 returns list of parsed events
• CodexWrapper.Exec.stream/2 returns lazy Stream of JsonLineEvent via Port
• Port-based streaming with line-by-line NDJSON parsing

Event types from codex --json

• thread.started, turn.started, item.completed, turn.completed, etc.

Acceptance criteria

• JsonLineEvent struct with parse/1 from JSON string
• JsonLineEvent accessor functions (event_type, data fields)
• Exec.execute_json/2 collects all events from --json run
• Exec.stream/2 returns lazy Stream via Port (like claude_wrapper's stream)
• Tests for event parsing

Depends on

• feat: Exec command and Result struct #2 (Exec command + Result)

Reference

• codex-wrapper (Rust): JsonLineEvent in exec.rs
• claude_wrapper: lib/claude_wrapper/stream_event.ex, Query.stream/2