chore: scaffold olsitec-foundation mono-repo

Repo topology, baseline overlay, planning docs (PLAN-001/002), ADR-004/005,
and the bootstrap/packages/documentation skeleton. Implementation (T00+) not started.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

documentation/decisions/ADR_004_layered_platform_foundation.md

@@ -0,0 +1,69 @@
+# ADR-004 — Layered Platform: `olsitec-foundation` Is a K8s-Free Layer 0
+
+**Date**: 2026-06-30
+**Status**: Accepted
+
+## Context
+
+We are building `olsitec-foundation` — the permanent, self-hosting technical foundation
+("the egg") for every future Olsitec product. Vision and detailed strategy:
+- [PLAN-001-forgejo.md](../PLAN-001-forgejo.md) (vision)
+- [PLAN-002-foundation-implementation.md](../PLAN-002-foundation-implementation.md) (strategy)
+
+PLAN-001 proposed deploying Forgejo **onto the existing Kubernetes cluster** via ArgoCD + Helm.
+But Kubernetes, ArgoCD, cert-manager and External Secrets Operator are themselves part of the
+platform the foundation is meant to *hatch*. A foundation that runs on them creates an
+unrecoverable circular dependency: disaster-recovery-from-nothing would first require rebuilding
+K8s+ArgoCD+ESO, which need git + an OCI registry + a secret store — which *are* the foundation.
+
+## Decision
+
+**Layer the platform.**
+
+- **Layer 0 — `olsitec-foundation` (the egg):** Forgejo (+ Actions + OCI/npm registry),
+  PostgreSQL, HashiCorp Vault, RustFS (S3), and a reverse proxy (Caddy) run as **plain OCI
+  containers on a single VM**, orchestrated by a **single Pulumi project** using the
+  `@pulumi/docker` provider over SSH. **No Kubernetes, no ArgoCD, no Helm at Layer 0.**
+- **Layer 1+ — everything else** (the existing olsicloud4 K8s platform, ArgoCD, Authentik,
+  Grafana/Prometheus, Longhorn, Renovate, internal PKI): a **consumer** of Layer 0. Its source
+  repos live in foundation-Forgejo, its CI runs in foundation-Actions, its images/charts in
+  foundation's registry, its secrets in foundation's Vault.
+
+Ratified sub-decisions:
+1. **Vault unseal:** Shamir + passphrase-gated unseal helper (no external KMS, no SaaS).
+2. **Object storage:** RustFS is the primary Layer-0 S3; the offsite backup replica is **non-RustFS**
+   so RustFS is never the only copy.
+3. **Offsite backup:** a second **self-hosted** location (different failure domain, no SaaS).
+
+The single external secret is the master passphrase (`PULUMI_CONFIG_PASSPHRASE`, passphrase
+secrets provider). Everything else is derived or generated by `@pulumi/random` into Vault
+(consistent with [ADR-002](ADR_002_pulumi_credential_lifecycle.md)).
+
+## Consequences
+
+**Easier**:
+- DR-from-nothing is genuinely `{VM + repo + passphrase}` — no prerequisite platform to rebuild.
+- Reuses existing Olsitec tooling: `pulumi/modules/docker` (Docker-over-SSH) and the
+  `olsitec-core/run.sh` Vault-init→capture-keys→passphrase-encrypted-config pattern.
+- Minimal moving parts at the root; the egg stays boring and inspectable.
+
+**Harder**:
+- Layer 0 is a single VM (SPOF) — mitigated by tested offsite DR (≤1h target), not HA.
+- ADR-002's `Pulumi → Vault → ESO → K8s Secret` chain applies only at Layer 1; Layer 0 consumers
+  are containers that read from Vault/rendered config directly.
+- Vault reboots require the passphrase for the unseal helper (auto-unseal deferred to Layer 1).
+
+## Alternatives Considered
+
+- **Forgejo on the existing K8s cluster (PLAN-001 literal):** rejected — circular DR dependency;
+  the egg cannot run on the chicken.
+- **Hybrid (bare Docker now, K8s-HA-ready later):** folded in — PLAN-001's K8s HA topology is
+  retained as the documented *future* HA path for Forgejo (PLAN-002 §8), not the bootstrap substrate.
+- **MinIO/Garage instead of RustFS at Layer 0:** rejected for now — RustFS matches the existing
+  credential flag; the S3 boundary keeps it replaceable if RustFS underperforms.
+
+## Confidence
+
+**High** — verified against existing source (`pulumi/modules/docker`, `pulumi/olsitec-core/run.sh`,
+`002_platform_architecture.md`) and ratified by the product owner on 2026-06-30. The one Medium-
+confidence area is RustFS production-readiness as primary S3 (flagged for later second-opinion).