feat(browser_mcp_agent): document and unblock local Ollama / OpenAI-compatible providers

Issue #329 reported that it's unclear how to point browser_mcp_agent at
a local Ollama backend. The mcp-agent framework already supports this
(Ollama exposes an OpenAI-compatible endpoint at
http://localhost:11434/v1), so the gap is documentation + one blocking
env-var check in main.py.

Changes:

- main.py: the hardcoded `os.getenv("OPENAI_API_KEY")` guard
  unconditionally rejected users who had correctly configured credentials
  via mcp_agent.secrets.yaml (both OpenAI and Ollama cases). Replace
  with a check that also accepts a present secrets.yaml, and update the
  error to point users at either path.

- mcp_agent.config.yaml: add a commented example showing the `openai:`
  block rewritten for Ollama (base_url + default_model). No behaviour
  change for existing OpenAI users.

- mcp_agent.secrets.yaml.example: add a commented Ollama example noting
  that any non-empty api_key works (Ollama doesn't authenticate).

- README.md: add a "Running with a local Ollama model" section, and
  reconcile the previously contradictory "export OPENAI_API_KEY" vs
  "use secrets.yaml" instructions into a single 'pick one' step.

Refs #329

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