[FEATURE]: Bump gitlab-ai-provider to 6.15.0 (adds GPT-6 Astra) #47791

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Describe the enhancement you want to request

GitLab published a new model, GPT-6 Astra, to its Duo model catalog. gitlab-ai-provider 6.15.0 adds the mapping for it (duo-chat-gpt-6-astra -> OpenAI gpt-6-astra, routed via the Responses API). opencode currently pins 6.14.0, so the model does not show up in the model picker for GitLab Duo users.

Bumping the pinned gitlab-ai-provider dependency in packages/core and packages/opencode from 6.14.0 to 6.15.0 makes the new model available. The corresponding models.dev entry is in anomalyco/models.dev#6499.

Benefit: GitLab Duo users get access to the current flagship OpenAI model (1,050,000 context / 128,000 max output) without waiting for an unrelated release. The change is a version bump only, no code changes needed on the opencode side since model resolution is driven by the provider package plus models.dev metadata.