Agent Teams: Add zellij terminal multiplexer support for split-pane mode

Feature Request

Agent Teams currently supports split-pane mode for tmux and iTerm2, automatically opening each teammate in its own pane. Zellij — a modern terminal multiplexer growing in popularity — has no integration, forcing users to fall back to in-process mode.

Why

Zellij is increasingly adopted as a tmux alternative due to its discoverability, sane defaults, and native floating panes. Users running Claude Code inside zellij should get the same split-pane experience as tmux users.

Implementation Guide

Zellij exposes everything needed via zellij action CLI commands, analogous to how tmux uses tmux split-window / tmux send-keys.

Detection

Zellij sets environment variables when running inside a session:

ZELLIJ=0                          # Present when inside zellij
ZELLIJ_SESSION_NAME=work          # Current session name
ZELLIJ_PANE_ID=3                  # Current pane ID

Detection is simply: process.env.ZELLIJ !== undefined

This mirrors the existing tmux detection (TMUX env var).

Spawning a teammate pane

# Split right with a command, named after the agent
zellij action new-pane \
  --direction right \
  --name "researcher" \
  -- claude --teammate-mode in-process ...

# Or split down
zellij action new-pane \
  --direction down \
  --name "implementer" \
  -- claude ...

Key flags:

• --direction right|down — controls split direction (like tmux split-window -h/-v)
• --name <NAME> — labels the pane (visible in zellij UI, no need for a separate rename step)
• --close-on-exit — auto-close pane when agent finishes (optional)
• --cwd <DIR> — set working directory
• -- <COMMAND>... — the command to run in the new pane

Alternative: new tab per agent

zellij action new-tab --name "researcher"
# Then the command runs in that tab

Alternative: floating panes (unique to zellij)

zellij action new-pane \
  --floating \
  --name "researcher" \
  --width 50% --height 50% \
  -- claude ...

This has no tmux equivalent and could be a zellij-specific bonus feature.

Writing to a pane

zellij action write-chars "some input"   # Types into the focused pane

Mapping to existing tmux integration

| tmux command | zellij equivalent |
|---|---|
| tmux split-window -h cmd | zellij action new-pane -d right -- cmd |
| tmux split-window -v cmd | zellij action new-pane -d down -- cmd |
| tmux send-keys "text" Enter | zellij action write-chars "text\n" |
| tmux select-pane -t N | zellij action focus-next-pane (or move-focus) |
| tmux rename-window "name" | --name flag on new-pane / new-tab |
| $TMUX detection | $ZELLIJ detection |

teammateMode setting

Add "zellij" as a valid value alongside "tmux", and update "auto" to detect $ZELLIJ:

auto priority: iTerm2 > tmux > zellij > in-process

Current workaround

claude --teammate-mode in-process

Works but loses the multi-pane visibility that makes the teams feature compelling.