`azd env unset` is documented but doesn't exist — doc bug or missing feature?

### Question

The environment-variables documentation tells users to remove a variable with `azd env unset VARIABLE_NAME`, but that command isn't recognized by the CLI. I'm not sure whether the **docs are wrong** or whether the **command was meant to exist and hasn't been implemented** — could you clarify the intended behavior?

**Doc page:** https://learn.microsoft.com/en-us/azure/developer/azure-developer-cli/manage-environment-variables#remove-or-update-variables

The "Remove or update variables" section says:

> To remove a variable from your environment: `azd env unset VARIABLE_NAME`

### What I observe

Running it fails with an unknown-command error:

```
azd env unset SOME_VARIABLE
```

Looking at `azd env --help`, the available subcommands are `set`, `set-secret`, `select`, `new`, `remove`, `list`, `refresh`, `get-values`, `get-value`, and the `env config` subgroup (`get`/`set`/`unset`) — there's no `unset` at the `azd env` level. The command is also absent from the [CLI reference](https://learn.microsoft.com/en-us/azure/developer/azure-developer-cli/reference). (`azd env config unset` exists but operates on `config.json`, not the environment's `.env` file, so it doesn't seem to be the same thing.)

### So which is it?

- **If it's a doc bug:** what's the supported way to remove a single variable from an environment's `.env` — is manually editing `.azure//.env` the intended approach?
- **If it's a missing feature:** is implementing `azd env unset ` on the roadmap?

### Environment

```
azd version 1.29.0 (commit fc06de978b9e9dc98a50ebb601a18b0f9f24071a) (stable)
```

Start with the linked environment-variables documentation, the CLI reference, and `azd env --help`; compare the documented `azd env unset VARIABLE_NAME` command with the available subcommands. Confirm whether `azd env config unset` affects `.env`, then clarify whether the documentation or the CLI behavior should change and document the supported outcome.

Written by the indexing model from the issue text.