feat: add MaxAttachments field to Model struct

[iceymoss]

Summary

• Add MaxAttachments int field to catwalk.Model struct (json:"max_attachments,omitempty")
• Set max_attachments: 10 for all Gemini models in gemini.json and vertexai.json

Motivation

Some AI providers enforce hard per-request limits on image attachments. For example, Google Gemini caps at 10 images per request — exceeding this makes sessions permanently unusable (see charmbracelet/crush#2604). Currently there is no way to express this limit in Catwalk's Model struct, forcing downstream consumers to hard-code provider-specific limits.

Changes

pkg/catwalk/provider.go

type Model struct {
    // ... existing fields ...
    SupportsImages         bool         `json:"supports_attachments"`
    MaxAttachments         int          `json:"max_attachments,omitempty"` // NEW
    Options                ModelOptions `json:"options,omitzero"`
}

Semantics:

• 0 (zero / omitted): no hard limit, rely on context window limits instead
• > 0: maximum number of image attachments allowed in a single request

internal/providers/configs/gemini.json

Add "max_attachments": 10 to all 6 Gemini models.

internal/providers/configs/vertexai.json

Add "max_attachments": 10 to all 6 Gemini models. Claude models on Vertex AI are left without the field (no known hard limit).

Known limits

Provider	Model	Limit	Source
Gemini	All models	10	Google docs
Vertex AI	Gemini models	10	Same as Gemini
Anthropic	All models	—	No hard limit (context window)
OpenAI	All models	—	No hard limit (context window)

Downstream impact

Once this ships, Crush can replace its hard-coded maxImagesForModel() switch with a simple read from model.CatwalkCfg.MaxAttachments, eliminating provider-specific logic from the consumer side.

Test plan

• go build ./... — passes
• go test ./... -count=1 — all existing tests pass
• JSON configs correctly deserialize with the new field

Closes #258