## Summary

`omarchy default agent <name>` fails for 8 of the 13 selectable agents. The command exits 1, prints a confusing mise error, and — importantly — **never writes the default agent file**, so the selection is silently not applied.

Affected: `pi`, `opencode`, `claude`, `codex`, `gemini`, `copilot`, `crush`, `cursor-agent`

Working: `omp`, `grok`, `muse` (via `agent_package`), `openclaw`, `hermes` (via `agent_installer`)

## Steps to reproduce

Pick any agent from the affected list that is not already installed as a non-mise binary in `~/.local/bin`:

```bash
omarchy default agent gemini
```

## Actual behavior

A floating terminal opens, mise fails, and the command exits 1:

```
mise ERROR  not found in mise tool registry
Could not install Gemini with mise
```

`~/.config/omarchy/defaults/agent` is left unchanged. The same failure occurs through the menu, since the menu action is literally `omarchy-default-agent <name>`.

## Expected behavior

Selecting an agent should set it as the default, and either install it or tell the user how to install it. Re-selecting an agent that is already the default should not fail.

## Root cause

`omarchy-default-agent` resolves an install through exactly two mechanisms: `agent_installer` (a dedicated script) and `agent_package` (a mise spec). Eight branches set neither, so both variables are empty:

```bash
# /usr/share/omarchy/bin/omarchy-default-agent
grok)   agent="grok";   name="Grok";   agent_package="npm:@xai-official/grok" ;;
gemini | gemini-cli) agent="gemini"; name="Gemini" ;;      # <-- no agent_package
claude | claude-code) agent="claude"; name="Claude Code" ;; # <-- no agent_package
```

The presence check and the install both then run mise with an empty spec:

```bash
agent_present() { user_install || mise where "$agent_package" &>/dev/null; }   # mise where ""
agent_install() { user_install || mise use -g "$agent_package"; }              # mise use -g ""
```

`mise where ""` and `mise use -g ""` always fail (`not found in mise tool registry`), and because the install check runs *before* the file write, the command exits without persisting anything:

```bash
if ! agent_install; then
  ...
  exit 1
fi

printf '%s\n' "$agent" >"$agent_file"   # never reached
```

## Why `user_install()` does not cover this

`user_install()` deliberately rejects mise shims:

```bash
# A mise copy would only shadow it, since the mise shims precede ~/.local/bin on PATH.
user_install() {
  [[ -x $HOME/.local/bin/$agent ]] &&
    { [[ -L $HOME/.local/bin/$agent ]] || ! grep -q '^mise use -g' "$HOME/.local/bin/$agent"; }
}
```

So for these 8 agents, mise is not a supported install path, and a non-mise install is required. That leaves two dead ends:

1. **Agent installed via mise** — `user_install()` returns false, `agent_package` is empty, both mechanisms fail. The default can never be set.
2. **Agent not installed at all** — same failure, and the error blames mise for an agent that was never meant to go through mise.

The result is that the "lazy-installed on first use" behaviour described for the default agent cannot work for these 8.

## Suggested fix

Either give each branch a `agent_package` (mise short names resolve fine — `mise search gemini` returns a valid entry), or fail early with an actionable message when neither mechanism is available:

```bash
if [[ -z "${agent_package:-}" && -z "${agent_installer:-}" ]] && ! user_install; then
  echo "$name is not installed. Install it yourself, then re-run this command." >&2
  exit 1
fi
```

A defensive guard in `agent_install()` against an empty `agent_package` would also prevent the confusing mise error regardless of which branch is hit.

## System details

- Omarchy 4.0.4-1
- mise 2026.9.12
- Reproduced on both a fresh selection and re-selecting an agent that is already the default