feat: add mc_stream tool for live job output streaming

[nigel-dev]

Description

Deferred from v1.5 release. Implement a mc_stream tool that provides live output streaming from running jobs.

Background

In v1.5, we enhanced mc_attach to auto-open tmux windows for serve-mode jobs and added structured mc_capture with event filtering. The next step is a dedicated streaming tool that provides real-time SSE event forwarding without requiring tmux attachment.

Requirements

• New mc_stream MCP tool
• Stream SSE events from serve-mode jobs in real-time
• Support event type filtering (file.edited, tool, error, all)
• Graceful connection lifecycle management
• Falls back to tmux capture-pane polling for TUI-mode jobs

Related

• Closes partially: originally part of feat: Rich job observability — structured telemetry, real-time dashboard, event logs, live streaming #66
• Builds on: feat: Migrate to serve-based orchestration with SDK monitoring and enhanced mc_attach #65 (serve-mode infrastructure)