Create and publish SourceOS build substrate repo

[mdheller]

Summary

Create SociOS-Linux/sourceos-build as the canonical SourceOS build substrate repository.

Why

We corrected the boundary: SourceOS should not become a monolithic platform repo. SourceOS build substrate should hold IaC, Tekton/Argo build orchestration, typed BuildRequest/BuildReceipt schemas, and artifact build receipts. socios remains the opt-in automation/orchestration commons that can drive this substrate.

Seed content to publish

Initial skeleton prepared in chat/sandbox includes:

• docs/adr/ADR-0001-sourceos-build-substrate.md
• schemas/sourceos/build-request.v0.1.schema.json
• schemas/sourceos/build-receipt.v0.1.schema.json
• example BuildRequest/BuildReceipt payloads
• Terraform skeleton
• Ansible skeleton
• Argo CD app skeleton
• Tekton pipeline skeleton

Acceptance criteria

• Create SociOS-Linux/sourceos-build
• Push initial skeleton on an import branch, not directly to main if branch protection is desired
• Add README explaining boundary: IaC/pipelines/schemas/receipts only, not product app/runtime
• Add schema validation task for examples
• Link from SociOS-Linux/socios docs or catalog once repo exists
• Open follow-up PR in socios to consume the BuildRequest/BuildReceipt schema paths

Non-goals

• Do not recreate a sourceos-platform monorepo
• Do not move agentplane, workstation-contracts, or socios responsibilities into this repo
• Do not embed secrets, registry credentials, or Foreman/Katello credentials