docs(readme): trim the quickstart API-key step to what the quickstart uses

[Birfy]

Why

Follow-up to #190, which merged the first draft of the API-key step. That draft answered questions nobody asks at the quickstart — a four-row base-URL table, what a CLI worker inherits, what doctor does and does not print — and ran 30 lines before the code block it was introducing. Length on a README is what makes a step get skipped, and every one of those details already lives on the agents page.

What changed

The section shrinks from 30 lines to 12, keeping only the path the quickstart itself takes:

• the two exports, with the "or GLM, a local vLLM, OpenAI itself" hint moved inline onto the base URL;
• the one sentence that says credentials come from the environment at call time and the base URL picks the provider;
• agentdescent doctor, and that the demo and pytest -q need no key;
• a link to Configuring your provider and key for the rest.

Dropped: the provider table, the ANTHROPIC_API_KEY route for claude (the quickstart's model is openai_compatible, so it is a second path a reader does not need at that point), the CLI-worker inheritance note, and the shell-profile line. The ### First, your API key heading becomes a bold lead-in — the README uses no other ###.

Net README.md: +7 / −26. No code changes.

Verification

tests/test_docs_links.py, tests/test_docs_examples.py and tests/test_api_docs.py pass (13 passed). The linked anchor resolves against the ## Configuring your provider and key heading in docs/agents.md.

🤖 Generated with Claude Code

https://claude.ai/code/session_018YNtrWn9WFYPgi3sWxEvSZ

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