docs: add Opper to the OpenAI-compatible base URL examples

[Felixkw12]

Name Opper in the README list of endpoints GRAFT_BASE_URL can point at, and add its base URL to the examples in .env.example. This documents the recipe offered when closing #419:

GRAFT_PROVIDER=openai GRAFT_BASE_URL=https://api.opper.ai/v3/compat GRAFT_API_KEY=<your-opper-key> GRAFT_MODEL=<model-id> graft build --deep

No code changes.

Disclosure: I work at Opper.

[trailhq-graft]

🌱 graft blast radius

Nothing outside this diff depends on it. 0 areas changed; no indexed dependents at depth 2.

⚠️ 2 changed files not in the graph (.env.example, README.md) — no parser claims the extension, or the index predates the file.

_{graft blast · origin/main...HEAD · depth 2 · 2 changed files}

Open the interactive graph → — click an area to see its dependent symbols at file:line.