chore: bump gitlab-ai-provider to 6.12.0

[vglafirov]

Issue for this PR

Closes #39095

Type of change

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

What does this PR do?

Bumps the pinned gitlab-ai-provider dependency from 6.11.1 to 6.12.0 in packages/core/package.json and packages/opencode/package.json, plus the resulting bun.lock update.

This release adds mappings for Claude Opus 5:

• duo-chat-opus-5 → claude-opus-5 (agentic chat)
• duo-workflow-opus-5 → claude_opus_5 (Duo Agent Platform workflow ref)

Provider release: https://gitlab.com/vglafirov/gitlab-ai-provider/-/merge_requests/33 (merged, published to npm as gitlab-ai-provider@6.12.0)

models.dev metadata for the model picker is a separate PR: anomalyco/models.dev#3765 (not yet merged, so duo-chat-opus-5 won't show up in opencode models until that lands and models.dev redeploys — this PR only bumps the provider dependency).

How did you verify your code works?

• bun install after the bump resolves gitlab-ai-provider@6.12.0 from the npm registry (not a file: link).
• bun turbo typecheck passes for all 30 packages in the monorepo (ran automatically via the pre-push hook).
• Ran opencode run -m gitlab/duo-chat-opus-5 "..." locally (with OPENCODE_MODELS_PATH pointed at a models.dev build containing the pending TOML entry) and confirmed:
  • A plain prompt gets a correct response from Claude Opus 5.
  • Tool calling works end-to-end (the model invoked ls and used the real output in its answer).

Screenshots / recordings

N/A - dependency bump, no UI change.

Checklist

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