Process leak when using custom LaunchAgent with KeepAlive

## Description

I encountered a process leak issue when running Colima with a custom LaunchAgent configuration. After investigating, I found the cause and wanted to share my findings in case it helps others.

## Environment

- macOS: Darwin 25.2.0 (Apple Silicon)
- Colima version: (installed via Homebrew)
- VM Type: vz

## The Problem

I had created a custom LaunchAgent plist at `~/Library/LaunchAgents/com.github.abiosoft.colima.plist` with the following configuration:

```xml
KeepAlive

```

When Colima failed to start (due to an unrelated disk attachment error), the `KeepAlive` setting caused launchd to continuously restart the process. However, each restart attempt spawned new `limactl` processes (especially `limactl usernet`) that were not properly cleaned up when the parent process exited.

Over time, this resulted in **3,800+ orphaned limactl processes** accumulating on my system, which caused:
- Extremely high system load (Load Average: 200-600)
- `kernel_task` consuming significant CPU managing these processes
- Overall system sluggishness

## Error Logs

From `/tmp/colima.err`, the startup was failing repeatedly:
```
time="2026-02-06T11:09:36+08:00" level=fatal msg="error starting vm: error at 'starting': exit status 1"
time="2026-02-06T11:09:47+08:00" level=fatal msg="error starting vm: error at 'starting': exit status 1`
... (repeated ~50 times in 10 minutes)
```

From `~/.colima/_lima/colima/ha.stderr.log`:
```
{"level":"fatal","msg":"failed to run attach disk \"colima\", in use by instance \"colima\"","time":"2026-02-06T11:19:54+08:00"}
```

## Resolution

I noticed that the official Homebrew service configuration uses a different KeepAlive setting:

```xml
KeepAlive

SuccessfulExit

```

After updating my LaunchAgent to use `SuccessfulExit` instead of a plain ``, and cleaning up the orphaned processes, the issue was resolved.

I also found that using `brew services start colima` with the official Homebrew plist works well and avoids this problem entirely.

## Investigation Notes

- The issue appears specific to using `KeepAlive: true` (boolean) versus `KeepAlive.SuccessfulExit: true` (dictionary)
- When startup fails repeatedly, the boolean form causes immediate restart without proper cleanup
- The `SuccessfulExit` form only restarts on clean exits, preventing the restart loop
- It might be helpful to document this behavior for users creating custom LaunchAgents

I was able to resolve this on my end, but wanted to report it in case it's helpful for the project or other users.