docs: document ANTHROPIC_BASE_URL and GOOGLE_GENERATIVE_AI_BASE_URL in .env.example

[kuishou68]

What

.env.example documents the optional OPENAI_BASE_URL endpoint override, but the analogous overrides for the other two providers are missing, even though the model adapter reads them:

• ANTHROPIC_BASE_URL — apps/server/src/engine/tanstack-agent.ts:35 (a trailing /v1 is stripped, because the Anthropic SDK adds /v1 itself)
• GOOGLE_GENERATIVE_AI_BASE_URL — apps/server/src/engine/tanstack-agent.ts:44 (a trailing /v1beta is stripped, because @google/genai adds the API version itself)

Why

Anyone routing Anthropic or Google traffic through a gateway/proxy endpoint has no way to discover these variables from the sample env file, and the suffix handling (/v1, /v1beta) is easy to get wrong silently. This adds two commented lines mirroring the existing OPENAI_BASE_URL entry, in the same style.

Docs-only change; no code touched.

Verification

• Grepped the source tree (excluding node_modules/dist): the only readers of these two variables are the adapter lines quoted above.
• pnpm exec biome check . still passes.