feat: extract Atlas as a reusable GCP production template

[rlancaster243]

Extracts the proven project-atlas/ implementation from the private DE-project-1 repository into a standalone, public, reusable GCP production-data-platform template.

Completed extraction

• imported the tracked Atlas subtree at the repository root
• removed the separate artifact-hosting product and nonessential raw evidence bundles
• preserved the Apache 2.0 license
• replaced personal identities and source-sandbox identifiers
• parameterized GCP projects, datasets, buckets, IAM/WIF identities, schedules, notifications, and cost ceilings
• adapted GitHub Actions, Cursor MCP, clean-clone paths, and acceptance tests for a standalone repository
• added adopter-oriented README, START_HERE, configuration, contribution, and security documentation
• retained sanitized reference evidence where architecture and regression gates require it
• added credentialless PR CI plus manually gated integration and deployment workflows
• removed all temporary migration and diagnostic workflows

Validation

Canonical atlas-ci run 29719000528 passed on head 1e80d7c:

• atlas-security-shell: PASS
• atlas-python: PASS
• atlas-dbt: PASS
• atlas-airflow: PASS
• atlas-ci-gate: PASS

The public-extraction scanner found zero forbidden-public patterns. Historical Atlas validation reports remain reference evidence only; each adopter must produce environment-specific deployment, incident, recovery, security, cost, and handoff evidence before making production-readiness claims.