When boards have variants available in micropython, the resulting build / filename is in the form <BOARD>-<VARIANT>-<date/version/etc>.hex

Similarly micropython has recently had a feature added to query on device what it's build is: micropython/micropython#16843
>>> sys.implementation._build will now return the build being run like <BOARD>-<VARIANT> for microcontroller ports (the variant is optional)

This PR modifies the build and clean cli commands to accept <BOARD>-<VARIANT> in the same format.

• Modify build and clean commands to use combined BOARD-VARIANT format instead of separate arguments
• Implement two-stage autocompletion: first suggest boards, then -VARIANT after a complete board name
• Update README.md to reflect new command format

In use this looks like:

I used Claude Code to assist with this change, starting from the prompt:

╭───────────────────────────────────────────────────────────────────╮
│ > the tool supports the build command in the form `mpbuild build  │
│   BOARD VARIANT` where `VARIANT` is optional. Instead I'd like    │
│   `mpbuild build BOARD-VARIANT` where the `-VARIANT` is still     │
│   optional. I want autocomplete to only suggest the BOARD         │
│   portion first, only when a full existing `BOARD` is on the      │
│   command line should another tab key press start suggesting      │
│   possible `-VARIANT` options.                                    │
╰───────────────────────────────────────────────────────────────────╯