auth switch silently writes to a stale path when config carries a persisted config: key

## Description

`doctl auth switch --context ` prints `Now using context [] by default` but **silently fails to persist** when the config file contains a top-level `config:` key that points somewhere other than the file actually in use. The switch writes the updated config to that *stale* path; the live config is never updated, and no error is shown.

This is self-inflicted: `writeConfig()` marshals `viper.AllSettings()`, which includes the bound `config` flag, so **every write bakes a top-level `config:` key into the config file**. It surfaces whenever a config is relocated or copied between machines — e.g. copying `~/Library/Application Support/doctl/config.yaml` from macOS into a Linux container's `~/.config/doctl/` — because the baked-in path no longer matches where the file now lives.

## Steps to reproduce

```bash
# Bake a stale config: path into the config (simulates a config copied from another machine)
sed -i 's#^config: .*#config: /tmp/doctl-STALE.yaml#' ~/.config/doctl/config.yaml
grep '^context:' ~/.config/doctl/config.yaml # -> context: default

doctl auth switch --context myteam
# prints: Now using context [myteam] by default

grep '^context:' ~/.config/doctl/config.yaml # STILL context: default (not persisted)
grep '^context:' /tmp/doctl-STALE.yaml # context: myteam (written to the stale path)
doctl account get # still the old context's account
```

## Root cause

1. `writeConfig()` ([`commands/auth.go`](https://github.com/digitalocean/doctl/blob/main/commands/auth.go)) does `yaml.Marshal(viper.AllSettings())`. `AllSettings()` includes `config` (bound via `BindPFlag("config", …)` in `commands/doit.go`), so a top-level `config: ` is persisted into the file.
2. On the next run, `viper.ReadInConfig()` loads that key. In viper's precedence a **config-file value outranks a flag's default**, so `viper.GetString("config")` returns the file's stored path, not the file in use.
3. `defaultConfigFileWriter()` uses that value as the write target (`os.Create(viper.GetString("config"))`). If the stored path is stale, the write lands there and the live file is untouched — while `RunAuthSwitch` has already printed success.

Passing `--config ` masks the bug, because an explicitly-set flag outranks the config-file value.

## Expected

`auth switch` should write to the config file doctl actually loaded (`viper.ConfigFileUsed()`), not to a path read back out of that file.

## Environment

- doctl 1.163.0 (also present on `main`)
- Reproduces on any OS; most likely to bite when a config is copied between machines.

## Related

Sibling of #1816 — same root cause (`writeConfig` persisting `viper.AllSettings()` keys), different symptom.

A fix + regression test is up in the PR linked below.

Start with commands/auth.go's writeConfig and defaultConfigFileWriter, then inspect the config flag binding in commands/doit.go and reproduce the stale-path steps. Add a regression test for auth switch and verify that the live config file's context changes instead of the stale path being written.

Written by the indexing model from the issue text.