[maui-labs docs] Document new `maui port check` CLI command

## Source PR

**PR**: https://github.com/dotnet/maui-labs/pull/200 — _feat: add maui port check command_
**Author**: @rmarinho
**Merged**: 2026-06-16

---

## Summary of Changes

PR #200 adds a new top-level `maui port check (port)` CLI subcommand to the `maui` global tool. The command is a cross-platform TCP port diagnostic utility that reports which process is listening on a given port number. It works on Windows (via P/Invoke `GetExtendedTcpTable`) and Unix/macOS (via `lsof`/`ss`/`netstat` fallback chain).

**User-facing additions:**
- New command group: `maui port`
- New command: `maui port check (port)`
- New JSON output model with `--json` support
- New error code `E1008` (`PortEnumerationFailed`)

---

## Documentation Pages Affected

| File | Change needed |
|------|--------------|
| `docs/developer-tools/cli/index.md` (or equivalent command reference) | Add `maui port` command group and `maui port check` subcommand |
| `docs/TOC.yml` | Add entry for port command if CLI commands are individually listed |

---

## Suggested Changes

### New section: `maui port check`

Add the following content to the CLI command reference under a new `## Port diagnostics` section (or alongside the `maui doctor` / diagnostics commands):

````markdown
## `maui port check`

Check which process is listening on a TCP port.

**Syntax**

```
maui port check (port) [--json]
```

**Arguments**

| Argument | Description |
|----------|-------------|
| `port` | TCP port number to check (1–65535). |

**Options**

| Option | Description |
|----------|-------------|
| `--json` | Output result as JSON. |

**Exit codes**

| Code | Meaning |
|------|---------|
| `0` | Port is free — no process is listening. |
| `1` | Port is in use — at least one process is listening. |
| `2` | Error (invalid argument or failed to enumerate ports). |

**Text output example — port free**

```
✔ Port 8080 is free.
```

**Text output example — port in use**

```
Port 8080 is in use:
PID 12345 (dotnet) 0.0.0.0 [ipv4]
```

**JSON output example — port in use**

```json
{
"port": 8080,
"in_use": true,
"listeners": [
{
"pid": 12345,
"process_name": "dotnet",
"address": "0.0.0.0",
"family": "ipv4",
"state": "listen"
}
]
}
```

**JSON output example — port free**

```json
{
"port": 8080,
"in_use": false,
"listeners": []
}
```

**Platforms**: Windows, macOS, Linux.

**Tip**: Use the `--json` flag together with an exit-code check in scripts to detect port conflicts before starting a DevFlow agent or development server:

```bash
maui port check 19223 --json
if [ $? -eq 1 ]; then echo "Port 19223 is already taken"; fi
```
````

### JSON schema reference (if the docs include API/output schemas)

Add `PortCheckResult` to any JSON output schema reference table:

| Model | Properties |
|-------|-----------|
| `PortCheckResult` | `port` (int), `in_use` (bool), `listeners` (array of `PortListenerResult`) |
| `PortListenerResult` | `pid` (int), `process_name` (string), `address` (string), `family` (`"ipv4"`\|`"ipv6"`), `state` (string, default `"listen"`) |

> Generated by [PR Documentation Check](https://github.com/dotnet/maui-labs/actions/runs/27622261154) for issue #200 · [◷](https://github.com/search?q=repo%3Adotnet%2Fdocs-maui+is%3Aissue+%22gh-aw-workflow-call-id%3A+dotnet%2Fmaui-labs%2Fpr-docs-check%22&type=issues)