Command-line argument parsing and help documentation for `deckcraft-cli` subcommands. Specifically, the `render` subcommand incorrectly interprets `--help` as an output file path.

**Environment:** deckcraft-cli 0.5.0, macOS universal zip (`deckcraft-cli-0.5.0-macos-universal.zip`), macOS on Apple silicon. The parser is unchanged at `main` (be14248), see below.

## What happens

`deckcraft-cli render --help` does not print help. It exits 0 and writes a 1920x1080 PNG named `--help` into the current directory:

```
$ deckcraft-cli render --help
--help (1920×1080)
$ echo $?
0
$ ls -la -- --help
-rw-r--r-- 1 charles wheel 42778 Oct 11 14:13 --help
$ file -- --help
--help: PNG image data, 1920 x 1080, 8-bit/color RGBA, non-interlaced
```

The image is slide 1 of a blank new document (no input file and no `--sample`, so `open()` falls through to `file.new`).

The other subcommands do not render anything, but none of them treats `--help` as a request for help either:

| Command | Result |
|---|---|
| `render --help` | writes `./--help` (PNG), exit 0 |
| `render -h` | `render needs an output path`, exit 1 |
| `info --help` | `--help: not a presentation DeckCraft can read` |
| `convert --help` | `convert needs IN and OUT` |
| `run --help` | `unknown option \`--help\`` (the right behaviour, just no help text) |
| `commands --help` | `[]` (filters the registry by the string "--help") |
| `deckcraft-cli --help` (top level) | prints the usage block, exit 1 |

## Cause

In `apps/deckcraft-cli/src/main.rs`, `render()` matches its arguments with a positional-input guard and then a catch-all:

```rust
other
if input.is_none()
&& !sample
&& !other....