fix(provider): default custom models to image input

[rekram1-node]

Summary

• default new custom models to text and image input with text output
• default legacy attachment support on while preserving explicit and inherited modalities
• lower V1 attachment settings into V2 capability modalities without adding attachment to core V2
• preserve neutral ModelV2.Info.empty() capabilities and document the temporary config-plugin default boundary

Issues

Fixes #20802
Fixes #30071

Addresses the image-input portion of #22673.

Same root cause / historical reports:

• [FEATURE]: Add supportsAttachments/vision capability for custom providers #8875
• [ModelScope] Custom Provider with Qwen Model Cannot Recognize Images - "Image Input Not Supported" #15040
• Bug: Custom provider models can't use image attachment (attachment flag not exposed in UI) #26271
• [Bug] Images silently discarded with local Ollama vision models + TUI lacks drag-and-drop #26780
• Incorrect image support configuration for XiaoMi mimo-v2.5 model #28614
• Gemma4 throu ollama -> vision capabilities #28768

Also addresses the custom-model image-default symptoms reported in these broader or mixed issues:

• Unable to read images for some models #5359
• Image Clipboard Paste Not Working in OpenCode #10154
• opencode cannot read images anymore. #25832
• [FEATURE]: Feature Request: Multimodal Support (Images & PDFs) via LM Studio #28677

Related modalities discoverability reports: #9897 and #25229. This PR removes the need to specify image modalities for the default custom-model case, but does not add documentation for explicit modality overrides.

Not covered here: terminal clipboard extraction, TUI drag-and-drop, Read/MCP tool-result media handling, PDFs, malformed image_url payloads, provider-side image errors, or genuinely text-only models.

Test plan

• bun test test/config/config.test.ts test/config/provider.test.ts test/model.test.ts in packages/core
• bun test test/provider/provider.test.ts in packages/opencode
• bun typecheck in packages/core
• bun typecheck in packages/opencode
• git diff --check