docs: document advanced configuration fields

[yzxlr]

Summary\n- add a dedicated configuration reference covering agents.list, bindings, routing, session identity links, Pico/IRC channel blocks, and tool limits\n- expand config.example.json with the advanced fields called out in issue #1489\n- fix the outdated multi-agent example in provider-refactoring docs and add a regression test that asserts the example config keeps these fields\n\n## Testing\n- jq empty config/config.example.json\n- go test ./pkg/config\n\nCloses #1489

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