Add OpenCode reviewer skills and agent configuration (#2880)
* Add OpenCode AI review for pull requests
Automated review via Cloudflare AI Gateway (Kimi K2.5) on every PR open
and push. Includes language-specific review skills for Go, Python, and
Rust, plus a cross-cutting Cog architecture skill.
Skip with the no-ai-review label or [skip-review]/[WIP] in the title.
* Fix model config: add capability flags for Kimi K2.5 via AI Gateway
The built-in cloudflare-ai-gateway provider needs explicit model
capabilities (tool_call, reasoning, interleaved, modalities, limits)
to construct a language model. Without them, the SDK threw
'sdk.languageModel is not a function'. Config matches the pattern
from cloudflare/ci/ai.
* Switch to custom OpenAI-compatible provider for AI Gateway
The built-in cloudflare-ai-gateway provider doesn't support Workers AI
models -- its SDK has no languageModel factory for them. Use a custom
provider (cf-gateway) with @ai-sdk/openai-compatible pointing at the
gateway's /compat endpoint instead. Construct CF_GATEWAY_BASE_URL from
secrets in CI.
* Fix: use vars not secrets for account/gateway IDs
GitHub variables use ${{ vars.* }}, not ${{ secrets.* }}. The account
ID and gateway ID were stored as repo variables, so the URL was being
constructed as /v1///compat.
* Remove OpenCode GHA workflows
