Turn off github workflows by default and add documentation around it

[TheEdgeOfRage]

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PR Summary

Low Risk
Primarily workflow-trigger and documentation changes; main risk is teams unintentionally having CI/CD disabled until they opt in by uncommenting triggers.

Overview
Disables automated dbt runs by default by commenting out the on: triggers in dbt_ci.yml (PR), dbt_deploy.yml (push/schedule), and dbt_prod.yml (schedule), leaving workflow_dispatch as the primary entry point.

Updates documentation (README.md, docs/cicd.md) to warn about Dune credit costs and to instruct teams to explicitly re-enable triggers by uncommenting the workflow sections; also includes minor formatting/clarity tweaks and adds an incremental-model run/test step in the PR CI workflow definition.

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