-a/--agent overwrites explicit pane commands in multi-window session configs, spawning duplicate agents

[celsobenedetti]

Summary

When workmux add is run with -a/--agent <name> on a session-mode config that defines multiple windows, the agent command overwrites the pane command of every window that doesn't already contain an agent pane — including explicitly configured editor/dev-server panes.

Example config:

mode: session
windows:
  - name: neovim
    panes:
      - command: nvim
  - name: agent
    panes:
      - command: <agent>
        focus: true

Run: workmux add my-task -a pi

Expected: window 1 runs nvim, window 2 runs pi.

Actual: window 1 also runs pi (a second agent) — nvim is gone.

Root cause

resolve_pane_configuration() in src/workflow/setup.rs runs per window whenever -a is passed. Its fallback logic replaces the focused/first pane's command with <agent> when no pane in that window "runs an agent":

if let Some(focused) = panes.iter_mut().find(|pane| pane.focus) {
    focused.command = Some("<agent>".into());   // clobbers explicit commands
    return panes;
}
if let Some(first) = panes.get_mut(0) {
    first.command = Some("<agent>".into());
    return panes;
}

The "agent" window is spared because its <agent> placeholder makes pane_runs_agent() return true. The "neovim" window is not — nvim is an explicit command, not an agent, so it gets overwritten.

This is particularly bad for multi-window configs where the user already designated which window runs the agent: -a should only affect <agent> placeholder resolution, never rewrite sibling windows' commands.

Reproduction

1. Config as above.
2. workmux add repro -a pi (or claude).
3. Inspect panes: tmux list-panes -a -F '#{pane_current_command}' → both windows run the agent.

Proposed fix (pseudo code)

Never overwrite an explicit pane command. Inject <agent> only into command-less (plain shell) panes or empty layouts:

fn resolve_pane_configuration(panes, config, agent):
    if agent is None:
        return panes
    if any pane in panes runs the agent (placeholder or known agent):
        return panes                       # layout already has an agent pane

    # prefer focused pane without a command, else first pane without a command
    target = first pane where (focus and command is None)
             else first pane where command is None
    if target exists:
        target.command = "<agent>"
        return panes

    if panes is empty:
        return [ { command: "<agent>", focus: true } ]   # default layout

    return panes                           # all panes explicit: leave untouched

Behavior change: with -a and a layout whose panes all have explicit commands, no agent is injected — the -a flag then only affects <agent> placeholder substitution and branch naming. Users who want an agent pane add <agent> (or a command-less pane) explicitly. The default (no panes/windows config) is unaffected: empty layout still gets a fresh agent pane.

Notes

• Present in v0.1.240 and latest master.
• Existing tests codifying the old behavior (agent_sets_focused_pane, agent_sets_first_pane_when_no_focus, preserves_named_profile_selector) need updating to assert explicit commands are preserved.
• Reproduced and verified locally; happy to open a PR with the fix + regression tests.