#1383 Add Atlas Cloud model provider support

Summary

• Add Atlas Cloud to the core model provider registry as an OpenAI-compatible provider using ATLASCLOUD_API_KEY, https://api.atlascloud.ai/v1, and the existing ATLASCLOUD_BASE_URL override convention.
• Add Atlas Cloud to the create-voltagent-app provider selection with default model atlascloud/qwen/qwen3.5-flash.
• Add focused registry/CLI coverage plus a changeset.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://voltagent.dev/docs/community/contributing/#commit-convention

Bugs / Features

• Related issue(s) linked (N/A: no related issue was opened)
• Tests for the changes have been added
• Docs have been added / updated
• Changesets have been added https://voltagent.dev/docs/community/contributing/#creating-a-changeset

What is the current behavior?

Atlas Cloud is not available in the core provider registry or the create-voltagent-app provider selection.

What is the new behavior?

Atlas Cloud is registered as an optional OpenAI-compatible provider and can be selected during app creation. Existing provider defaults remain unchanged.

fixes (issue): N/A

Notes for reviewers

• OpenRouter and the other existing provider paths remain unchanged.
• The changeset releases @voltagent/core and create-voltagent-app as patch updates.
• The current AI review findings were addressed in 933f42b.

Validation

• corepack pnpm install --frozen-lockfile --ignore-scripts
• corepack pnpm --filter @voltagent/internal build
• corepack pnpm --filter @voltagent/core exec vitest run src/registries/model-provider-registry-atlascloud.spec.ts src/registries/model-provider-registry-minimax.spec.ts (12 passed)
• corepack pnpm --filter create-voltagent-app exec vitest run src/cli.integration.spec.ts (1 passed)
• corepack pnpm --filter @voltagent/core typecheck
• corepack pnpm --filter @voltagent/core build
• corepack pnpm --filter create-voltagent-app build
• corepack pnpm exec biome check packages/core/src/registries/model-provider-registry.ts packages/core/src/registries/model-provider-registry-atlascloud.spec.ts packages/create-voltagent-app/src/types.ts packages/create-voltagent-app/src/cli.ts packages/create-voltagent-app/src/cli.integration.spec.ts
• corepack pnpm exec prettier --check .changeset/atlascloud-provider.md
• git diff --check

The config portal full typecheck still reports unrelated pre-existing AddAgentFlow / AddGatewayFlow errors. No README, sponsor, logo, credits, or partner promotion was added.