feat(text chat): expose image input via repeatable --image (multimodal M3, multi-image)

[shoemoney]

Closes #224.

What

mmx text chat now takes a repeatable --image <path-or-url>:

# single image
mmx text chat --image ./photo.jpg --message "What breed is this dog?"

# multiple images in one call
mmx text chat --image ./before.png --image ./after.png \
  --message "List every visual difference between these two."

Why

M3 is multimodal and already accepts multiple images through --messages-file, but there was no CLI path to it — you had to hand-write a base64 messages JSON. Worse, the documented OpenAI shape ({"type":"image_url", ...}) is rejected, because text chat posts to the Anthropic-compatible /anthropic/v1/messages endpoint and needs {"type":"image","source":{"type":"base64",...}}. --image emits the right shape for you.

How

• toImageBlock() in src/utils/image.ts wraps the existing toDataUri() (local paths, http(s) URLs, and pre-made data URIs all work) and returns an Anthropic image block.
• src/commands/text/chat.ts appends the blocks to the last user message, promoting content from a string to a block array. Text goes first so the model reads the instruction before the pixels. With no --message, the images become the user message.
• ContentBlock gains the image variant.
• When --image is present and --model is not, the model resolves to MiniMax-M3 — otherwise a text-only defaultTextModel in config would silently break every image request. An explicit --model still wins.
• Docs updated: README.md, README_CN.md, and skill/SKILL.md (which also now calls out the OpenAI-vs-Anthropic block-shape gotcha).

Verification

Dry run of the built binary:

$ mmx text chat --image a.png --image b.png --message "diff these" --dry-run --output json
{
  "request": {
    "model": "MiniMax-M3",
    "messages": [ { "role": "user", "content": [
      { "type": "text",  "text": "diff these" },
      { "type": "image", "source": { "type": "base64", "media_type": "image/png", "data": "..." } },
      { "type": "image", "source": { "type": "base64", "media_type": "image/png", "data": "..." } }
    ] } ],
    "max_tokens": 4096,
    "stream": false
  }
}

That is byte-for-byte the payload shape I confirmed M3 answers correctly in #224.

Six new tests in test/commands/text/chat.test.ts cover block shape, multi-image, image-without-message, the model override, explicit --model winning, and the missing-file error. bun test 455 pass / 0 fail; bun run typecheck and bun run lint clean.

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