v1.2.0

feat: MCP server + pluggable runners + macOS builds — v1.2.0

- hivemux mcp: stdio JSON-RPC MCP server (9 tools) so a conductor agent drives a
  fleet — spawn_agent, start_loop (fire-and-poll), get_status, usage, conflicts,
  merge, kill, broadcast. Safety: per-agent cost cap, max-concurrent, acceptEdits.
  Protocol verified deterministically (initialize / tools/list / tools/call).
- pluggable runners (core/runners.ts): loops aren't claude-only. --runner; claude
  built-in + verified, codex/gemini/OpenRouter via ~/.hivemux/config.json runners.
- macOS: release workflow now builds a .dmg + hivemux-macos-arm64 (code already
  portable: Bun + tmux + node: APIs).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>