feat: cmux as configurable backend for session management

[abhirup-dev]

Summary

Adds cmux as a configurable backend alternative to the agent-deck CLI daemon. cmux is a native macOS terminal app purpose-built for AI agent orchestration, with GPU-accelerated rendering (libghostty), integrated browser, and rich sidebar metadata.

With this change, sessions can run in cmux's native UI instead of Neovim terminal buffers, while maintaining full feature parity with the existing backend.

Architecture

UI Layer (picker, parallel, send, output)
                    |
         require("agent-deck.backend")   <-- NEW dispatch layer
                    |
       +------------+------------+
       |                         |
  backend/               backend/
  agent_deck.lua         cmux.lua
  (wraps cli.lua)        (cmux CLI)
       |                         |
  agent-deck daemon       cmux app
  (Rust + tmux)           (macOS native)

What changed

New files (7):

• backend.lua — dispatch layer with init(), name(), method forwarding for all 15 interface methods
• backend/agent_deck.lua — thin cli.lua wrapper + focus_session no-op (preserves existing behavior exactly)
• backend/cmux.lua — full cmux implementation: launch, list, send, output, start/stop/restart/delete, groups, focus
• backend/cmux_status.lua — Claude .jsonl tail parser for session status detection (sub-millisecond)
• backend/ansi.lua — ANSI escape sequence stripping for clean read-screen output
• claude_paths.lua — extracted shared path utilities from picker.lua
• session_cmd.lua — extracted shared build_cmd logic previously duplicated between picker.lua and parallel.lua

Modified files (7):

• init.lua — backend.init(opts.backend) in setup; all cli refs swapped to backend; sync timer gated
• persist.lua — added _cmux_sessions accessors (get/set/remove/all)
• ui/picker.lua — uses shared modules; adds focus_session path for cmux
• ui/parallel.lua — uses shared modules; cmux-aware layout delegates to focus_session
• ui/send.lua, ui/output.lua, ui/info.lua — mechanical cli → backend swap

Feature parity

Operation	agent-deck backend	cmux backend
Launch session	agent-deck launch	cmux new-workspace + new-split + send-surface
List sessions	agent-deck list --json	cmux list-surfaces + persist metadata + .jsonl status
Session status	Daemon-reported	.jsonl tail parse (Claude) / surface-alive (other)
Send input	agent-deck session send	cmux send-surface + send-key enter
Read output	agent-deck session output	cmux read-screen + ANSI strip
Stop/Delete	CLI commands	send-key ctrl-c / close-surface
Groups	agent-deck group commands	cmux workspaces
Focus session	N/A (Neovim terminals)	cmux focus-surface + select-workspace

Configuration

require("agent-deck").setup({
  backend = "cmux",  -- "agent-deck" (default) | "cmux"
})

Known limitations (cmux backend)

• macOS only
• No process exit detection API
• group_move is persist-only (cmux doesn't support cross-workspace moves)
• read-screen output is ANSI-stripped raw text vs structured JSON
• No SSH/remote (future scope)

Review fixes applied

1. Critical: session_delete use-after-delete — fixed
2. Medium: new-workspace missing --name flag — fixed
3. Medium: stdout pipe leak on spawn failure — fixed
4. Medium: Duplicated path logic — fixed by importing shared module

Test plan

• Default backend: no errors in :AgentDeckLog, statusline updates
• Dan, Dap, Dal, DaR all work (session lifecycle)
• 2 sessions in same dir — no clobbering
• Das + Dao (send/output)
• Picker actions: stop, restart, delete
• backend = "cmux": health check, launch, focus, send, output

🤖 Generated with Claude Code