feat(ui): config-generator AI-provider-mode field group

[JSONbored]

Add the AI-provider-mode field group to the config generator (#1683): provider mode selection (single / consensus / synthesis and provider/model) plus a clear secret-handling notice. One field-section; the generator must NOT capture or serialize secret values (only mode/model names), preserving the public/private boundary.

Deliverables

• Provider-mode selector + model field group reusing patterns from ai-review-settings.tsx:39
• Explicit inline notice that API keys are set via env/secrets, not this form (no secret input rendered or stored)
• Mode/model values write into GeneratorFormState under the existing config schema keys (private-config.ts)
• Callout primitive (primitives.tsx:243) for the secret-handling note
• Vitest test: each mode option, model field edit, and assert no secret field exists in the emitted state

References

• apps/gittensory-ui/src/components/site/app-panels/ai-review-settings.tsx:39
• apps/gittensory-ui/src/components/site/primitives.tsx:243
• apps/gittensory-ui/src/routes/docs.self-hosting-ai-providers.tsx
• src/selfhost/private-config.ts

Part of #1683.

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📸 Required: before/after screenshots (this is a UI change)

Non-negotiable. PRs that change UI must include a before/after screenshot table. PRs without it are automatically closed — no exceptions.

Rules:

• Show every page/feature you changed — before and after, one row each.
• Use the markdown table below. Screenshots go inside the table only — no large full-size images pasted elsewhere in the PR.
• Each cell is a clickable thumbnail (a small image linking to the full-size image) with a one-line caption underneath.
• Upload screenshots to the PR by drag-and-drop (GitHub hosts them on its CDN). Do NOT commit image files to the repo.

Copy this and fill it in (replace FULL_IMAGE_URL with the URL GitHub gives you after you drop the image in):

| Page / Feature | Before | After |
|---|---|---|
| `/your-route` | [<img src="FULL_IMAGE_URL" width="260">](FULL_IMAGE_URL)<br><sub>before: short caption</sub> | [<img src="FULL_IMAGE_URL" width="260">](FULL_IMAGE_URL)<br><sub>after: short caption</sub> |

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size: S · held for later release (gittensor:feature once activated)

[linear-code]

JSO-2309