chore: bump gitlab-ai-provider to 6.18.0

[vglafirov]

Issue for this PR

Closes #50889

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Bumps the pinned gitlab-ai-provider to 6.18.0 in both packages that declare it on dev:

• packages/opencode: 6.15.0 → 6.18.0. This is the v1 app, and the previous bump (maint(gitlab): bump gitlab-ai-provider to 6.16.0 #50742) missed this pin.
• packages/core: 6.16.0 → 6.18.0.

bun.lock now resolves a single gitlab-ai-provider@6.18.0. That removes the separate opencode/gitlab-ai-provider@6.15.0 entry and its nested openai/zod copies.

6.18.0 adds the duo-chat-gpt-6-sol / duo-chat-gpt-6-luna mappings. packages/opencode also picks up 6.16.0's Claude Opus 5.5 mappings and project-scoped workflow discovery, and 6.17.1's corrected VERSION (it previously reported the prior release in the User-Agent). The API we use is unchanged: createGitLab, VERSION, isWorkflowModel, discoverWorkflowModels and GitLabWorkflowLanguageModel. 6.17.0 only added an optional cacheKey option, so no source changes were needed.

How did you verify your code works?

• bun install: both workspace packages resolve gitlab-ai-provider 6.18.0 from npm, not a file: link.
• bun typecheck passes in packages/core and packages/opencode.
• packages/opencode: bun test test/provider/gitlab-duo.test.ts test/provider/provider.test.ts test/provider/transform.test.ts gives 663 pass, 0 fail.
• packages/core: bun test gives 1098 pass, 0 fail.
• Running a single packages/core/test/plugin/* file on its own fails with Cannot access 'GitLabPlugin' before initialization. That also happens on unmodified dev and is unrelated to this bump.

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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