Add config conventions

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Summary

• add narrow config helpers for env, object, and named profile loading
• return structured config diagnostics for missing and malformed values
• document predictable config load order and add a config example CLI
• archive the OpenSpec change into canon after implementation

Validation

• npm run verify
• openspec validate add-config-conventions
• openspec validate --all

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Superseded by #6, which landed the stacked config-conventions and testing-fixtures work together.