fix: use Anthropic SDK for comment classification instead of hardcoded API URL

[the-real-mathew-moon]

Summary

• Replace raw fetch() call to hardcoded https://api.anthropic.com/v1/messages in post-buffered-inline-comments.ts with the official @anthropic-ai/sdk client
• The SDK automatically respects ANTHROPIC_API_KEY and ANTHROPIC_BASE_URL environment variables, fixing the issue for users routing traffic through a proxy or custom gateway
• Pass ANTHROPIC_BASE_URL env var to the post-buffered-inline-comments step in action.yml

Motivation

The classifyComments() function used a hardcoded https://api.anthropic.com/v1/messages URL via raw fetch(), bypassing ANTHROPIC_BASE_URL. Users who set this env var to route traffic through an LLM gateway or proxy found that the comment classification step always hit api.anthropic.com directly.

Changes

File	Change
src/entrypoints/post-buffered-inline-comments.ts	Replace raw fetch() with new Anthropic().messages.create()
action.yml	Pass ANTHROPIC_BASE_URL to the post-buffered-inline-comments step
package.json / bun.lock	Add @anthropic-ai/sdk dependency

Note on src/github/token.ts

The other hardcoded api.anthropic.com usage in token.ts (/api/github/github-app-token-exchange) is a proprietary Anthropic infrastructure endpoint for OIDC token exchange — not a standard Messages API call. It cannot be replaced with an SDK call, and making it respect ANTHROPIC_BASE_URL could break functionality (e.g., Bedrock/Vertex proxies wouldn't have this endpoint).

Test plan

• bun run typecheck passes
• bun run format:check passes
• bun test — all 653 tests pass

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