label Agy's model field for the agent it selects, and allow a per-render override

[atomantic]

Summary

The Image Gen settings field for the Agy (Antigravity) backend was labeled "Image model" while listing Gemini agent models (gemini-3.6-flash-high, gemini-3.1-pro-high, …), which reads as the wrong catalog being loaded.

It isn't the wrong catalog — the label was wrong. agy models enumerates the agent models the CLI's --model flag accepts, and that agent is what drives the built-in generate_image tool.

Agy exposes no knob for the underlying image model. Verified against the shipped binary: GenerateImageToolConfig carries only force_disable and output_directory, and the tool args are prompt / image name / aspect ratio / reference paths. There is no imagen-3 / imagen-3-fast option to offer, and handing --model an id outside the agy models catalog makes agy exit non-zero before it generates. So the field is now "Default agent model" with that explanation, rather than a label promising a knob the CLI doesn't have.

The override half of the request is delivered in full:

• New cloudModel request field, folded into the provider's own model by resolveCloudProviderConfig and stripped before it reaches persisted job params.
• Gated per provider by a supportsModelOverride spec flag — grok is false (fixed xAI image backend, no model knob), so an override there is ignored rather than silently changing nothing at spawn time. Mirrored client-side as supportsCloudModelOverride so the two stay in lock-step.
• The Image Gen page renders an Agent model picker driven by the live probe, so a single queue item can run on a different agent model than the saved default without a settings round-trip. Blank inherits the Settings default (and the option names what that resolves to). A pin the catalog no longer lists falls back to the default instead of failing the render — derived, not written back to state, so a transient probe failure doesn't destroy the selection.
• The probe is opt-in via an enabled flag on the new useAgyModels hook, so a backend the user hasn't selected never spawns a child process.

Also fixes a latent bug found along the way: the retry editor on a failed Agy job was bound to params.modelId (a local image-model registry id Agy never reads), so the model was effectively uneditable on retry. It now follows params.model for every backend that takes a CLI model, and renders a configured-default sentinel as an empty field so an untouched form doesn't submit model: ''.

Test plan

• cd client && npm run lint — clean
• cd client && npx vitest run — 478 files / 5349 tests pass
• cd server && npx vitest run routes/imageGen.test.js services/imageGen/ lib/antigravity lib/providerModels lib/index.test.js — 15 files / 392 tests pass

New coverage:

• server/routes/imageGen.test.js — cloudModel overrides the saved model for codex and agy, does not leak the raw field into persisted job params, an omitted value leaves the saved default in place, and a shell-unsafe id is rejected by Zod before settings are ever read.
• server/services/imageGen/cloudProviderConfig.test.js — override wins over the saved default; blank/whitespace/non-string means "inherit"; an override for grok is ignored and adds no model to jobParams.
• client/src/components/imageGen/ImageGenControls.test.jsx — the picker is hidden when the probe returned nothing or the host wired no handler, lists every probed model plus a named "Settings default (…)" option, and emits blank when cleared.
• client/src/hooks/useAgyModels.test.jsx — no probe while disabled, silent request, in-body error vs rejected request both surface, non-array models degrades to [], refresh() re-probes.
• client/src/components/media/MediaJobsQueue.test.jsx — an Agy retry pre-fills from params.model and submits the edit; a configured-default sentinel shows as empty and sends no override.

The 53 failing server suites on this machine (pipeline / universeBuilder / writersRoom / catalog / sharing) are pre-existing — portos_test isn't provisioned here, and they fail identically on a clean main checkout. None of them touch image gen.