feat: add Zellij as alternative terminal multiplexer

[pmarsceill]

Summary

• Add support for Zellij as an alternative to tmux for managing agent sessions
• Users can configure their preferred multiplexer via config file, environment variable, or default to tmux
• Implements multiplexer abstraction interface for clean separation of concerns

Changes

• New files:

  • internal/daemon/multiplexer.go - Interface definition and factory
  • internal/daemon/multiplexer_tmux.go - Tmux implementation
  • internal/daemon/multiplexer_zellij.go - Zellij implementation

• Modified files:

  • internal/daemon/process.go - Refactored to use Multiplexer interface
  • internal/daemon/server.go - Initialize multiplexer from config
  • internal/cli/config.go - Added multiplexer default
  • internal/cli/root.go - Added getMultiplexer() helper
  • internal/cli/up.go - Pass multiplexer to daemon config
  • internal/cli/agent_watch.go - Support both multiplexers with appropriate keyboard hints
  • internal/cli/clean.go - Clean sessions from both multiplexers
  • proto/map/v1/daemon.proto - Added multiplexer field to status messages
  • README.md - Documentation for multiplexer configuration

Configuration

The multiplexer can be configured via (in priority order):

1. Environment variable: MAP_MULTIPLEXER=zellij
2. Config file: ~/.mapd/config.yaml with multiplexer: zellij
3. Default: tmux

Test plan

• Verify tmux still works as default
• Test with MAP_MULTIPLEXER=zellij map up
• Test map config set multiplexer zellij
• Test map agent watch shows correct keyboard shortcuts for each multiplexer
• Test map clean removes sessions from both multiplexers
• Run make test and golangci-lint run

Closes #14

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