Replace per-setting configuration files with config.yaml

[dapi]

Summary

• replace legacy per-setting file reads with strict project and user config.yaml documents
• preserve CLI, project, user, environment, and built-in precedence with source reporting
• document prompt file references, the full YAML schema, and the breaking-release decision

Verification

• GOCACHE=/private/tmp/code-converge-go-cache go test ./internal/config
• GOCACHE=/private/tmp/code-converge-go-cache go vet ./...
• make docs-lint
• git diff --check

go test ./internal/app cannot execute in this sandbox because the macOS loader aborts generated test binaries with missing LC_UUID load command.