[Portal]: useForHarness checkbox in the AI integration form

[07prajwal2000]

Why

useForHarness decides whether an AI integration may drive an agent harness conversation. The whole path exists except the one control that sets it:

• the flag is in the config schema for all five AI variants, defaulting to false (apps/server/src/api/v1/integrations/schemas.ts:180,188,196,204,217)
• the gate reads it (apps/ai-gateway/src/api/v1/harness-conversations/send-message/service.ts:26)
• the filtered list endpoint exists: GET /api/v1/integrations/:projectId/list-basic?useForHarness=true (get-basic-list)
• the portal already calls that endpoint with the filter (apps/portal/src/services/integrations.ts:25, query/integrationsQuery.ts:21)

Nothing can set it to true. Two comments in the codebase already say so:

• schemas.ts:174 - TODO(ui): build a toggle in the AI integration form to flip useForHarness.
• send-message/service.ts:18 - ...and there is no UI yet to flip useForHarness.

Result: the harness falls back to the project's configured default agent, and a user cannot opt any other AI integration into the pool.

What

Add a useForHarness checkbox to the AI connector form in the portal: apps/portal/src/components/integrations/connectors/AiForm.tsx.

• AI group only. Database, KV, and observability configs have no such field and must not grow one.
• Applies to every AI variant the form serves: OpenAI, Anthropic, Gemini, Mistral, and OpenAI-Compatible (showBaseUrl).
• Defaults to unchecked - the schema default is false and nothing should become harness-eligible implicitly.
• Reads the current value on edit and persists it on create and update, like the other config fields via setField.
• Use Checkbox from @fluxify/components, not a raw HeroUI checkbox (see AGENT.md - the shared one is theme-aware and self-contained).
• Give it a short description saying what it does: this integration can be selected to run agent harness conversations.

Remove the TODO(ui) comment at schemas.ts:174 and correct the stale "there is no UI yet" sentence in send-message/service.ts:18 once it ships.

Out of scope

Any change to how the harness picks a model when several integrations are eligible. This issue only exposes the flag.

Done when

• An AI integration can be created and edited with useForHarness on or off from apps/portal.
• The value round-trips: saved, reloaded on edit, and reflected in list-basic?useForHarness=true.
• A newly created AI integration is not harness-eligible unless the box was ticked.
• Non-AI integration forms are unchanged.
• bun run --cwd apps/portal lint passes.