feat(core): make image generation a Code Mode tool - #60086

image_generate was a direct tool, offered next to execute on every request where an image model exists. I moved it into Code Mode, unpinned, so it sits in the catalog until an agent needs it, and the prompt stays smaller on every turn that doesn't generate images.

It gets its own image namespace rather than opencode, since opencode is about managing OpenCode itself (sessions, worktrees, models). That also keeps it clear of the internal plugin's opencode_image_generate.

 tools offered to the model
   execute
-  image_generate            (direct, every request)
 code mode catalog
+  image (Generate and edit images with the image models of your connected providers.)
+    generate                (tools.image.generate, not pinned)

The tool's effective name stays image_generate, so permission rules ("image_generate": "deny"), background jobs and restart recovery are unchanged. It's still registered only while some connected provider has an image model, and it re-checks when the catalog changes. With Code Mode denied it falls back to a direct image_generate tool like the other Code Mode tools.

Verification

• packages/core test/tool-image-generate.test.ts (15 pass, twice): the tool shows up in the Code Mode catalog as image.generate, isn't pinned and isn't a direct tool; it appears and disappears with image models; a new test calls tools.image.generate through execute. The other tests run it with Code Mode denied, where it's direct. test/lib/tool.ts's executeTool takes optional permissions for that.
• session-execution, codemode and tool-schedule tests pass (45).
• bun run typecheck:tsc in packages/core and bun run check pass.