feat(opencode): support MCP resource content

[rekram1-node]

Summary

• resolve MCP resource mentions without forwarding custom resource URIs as provider-downloadable files
• deliver supported image blobs as native image attachments and describe other binary resources by MIME type and size
• add model-callable list_mcp_resources and read_mcp_resource tools
• share resource content conversion between user-selected resources and model-called reads

Behavior

resources/read text is injected as text. Supported image blobs are converted from the returned base64 bytes into data-backed image parts. Other blobs remain visible as neutral binary resource descriptions. The original MCP URI is retained only as resource provenance.

The new tools let models discover concrete resources across connected MCP servers and read a resource by server name and URI.

Validation

• bun test test/mcp/content.test.ts test/tool/mcp-resource.test.ts test/tool/registry.test.ts test/session/message-v2.test.ts
• manually verified the local image/png MCP fixture becomes an image attachment containing the exact 14,396 returned bytes

No broad typecheck was run.