Add ADR-015 (control/AI-worker host ubongo)

docs/decisions/015-control-host.md

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+# ADR-015 — Control / development / AI-worker host (`ubongo`)
+
+## Context
+
+Earlier ADRs framed the control node — the host that runs Terraform and Ansible —
+as a **single Debian 13 VM on the Proxmox cluster**, manually provisioned as the one
+documented exception to "Terraform owns VM existence" (ADR-009). That framing treats
+the control node purely as a control-plane runner.
+
+It fails four needs, all confirmed as drivers:
+
+1. **Cold-start bootstrap** — the VM that runs Terraform/Ansible cannot exist until
+   something else creates it; the bootstrap is circular and awkward.
+2. **Always-on availability** — the operator wants to SSH in from a work PC or
+   anywhere to drive Claude Code. A cluster VM is gone whenever the cluster is down
+   or being rebuilt.
+3. **Recovery / disaster** — the tool used to rebuild the cluster must not live
+   inside the thing it rebuilds.
+4. **Dev ergonomics** — a persistent home for Claude Code + the repo, not entangled
+   with production VM lifecycle.
+
+A laptop-only answer fails always-on and recovery. A VM-only answer fails cold-start
+and recovery. A small dedicated always-on physical machine outside the cluster
+satisfies all four.
+
+## Decision
+
+Introduce **`ubongo`** (Swahili: *brain*, consistent with the fleet's theme): a
+single dedicated x86-64 mini-PC, always-on, living **outside** the Proxmox cluster.
+It becomes *the* control node and collapses four roles into one box:
+
+- Terraform + Ansible runner (control plane)
+- Claude Code / AI-worker host the operator SSHes into
+- Local test runner (Molecule/Docker, lint, and later a browser stack)
+- Persistent dev home for the repo
+
+There is **no longer a control VM on the cluster.** The `control` inventory group
+points at this physical box. This *strengthens* the ADR-009 control-node exception:
+it is genuinely outside Terraform's world, not a VM pretending to be the exception.
+Every other host stays a Terraform-managed VM exactly as designed.
+
+`ubongo` runs **plain Debian 13** (the `base` role applies). It is not a hypervisor
+and runs no `docker_host` services.
+
+### Hardware target
+
+| Spec | Target | Why |
+|---|---|---|
+| CPU | 4 cores, x86-64 (Intel N100-class or better) | Molecule containers + Chromium prefer x86 |
+| RAM | 16 GB | Docker + headless Chromium + toolchain headroom |
+| Disk | 250 GB SSD/NVMe | Docker images, molecule layers, repos, browser cache |
+| Network | Wired GbE | Always-on reliability over Wi-Fi |
+| Power | Low draw (≤15 W idle) | Runs 24/7 |
+
+Indicative: a refurb Dell/Lenovo/HP micro (USFF) or an N100 mini-PC (~€150–250).
+Claude Code itself is light (the model runs in Anthropic's cloud); the sizing driver
+is **all testing being local** — Molecule (Docker), lint, and a future
+headless-Chromium/Playwright stack.
+
+### Provisioning (bootstrap path)
+
+Manual, on bare metal:
+
+1. Install Debian 13 on the box (one-time, by hand).
+2. `git clone` the repo; `make setup`; `make collections`; set up `rbw` + unlock.
+3. Join the mesh VPN (choice deferred — see below).
+4. From then on `ubongo` manages every other host normally; Ansible manages *it* for
+   baseline config via the `control` group (`base` role only).
+
+### Access & security
+
+- Remote access is via the **mesh VPN** (choice deferred). SSH to `ubongo` over the
+  mesh; nothing is published to the public internet — this stays inside ADR-002.
+- `ubongo` runs the `base` role: SSH hardening, nftables default-deny, fail2ban,
+  auditd, unattended-upgrades. Inbound SSH is allowed **only on the mesh interface**,
+  denied on the physical NIC.
+
+### Recovery model
+
+`ubongo` is the rebuild tool, so three things must survive a full cluster loss:
+
+1. **`mamba` (laptop) is a break-glass clone** — repo + toolchain + mesh + `rbw`,
+   able to drive the fleet if `ubongo` dies.
+2. **Terraform state** lives on `ubongo`, backed up encrypted off-box (synced to
+   `mamba`). For a 2–5 VM fleet it is also reconstructable via `terraform import`.
+3. **Vault password** — `ubongo` gets it from Vaultwarden via `rbw`. `rbw` keeps a
+   local encrypted copy of the vault and decrypts it offline with the operator's
+   Vaultwarden master password, so `ubongo` can decrypt the Ansible vault with the
+   whole cluster down — provided `rbw` has synced once and the operator keeps the
+   Vaultwarden master password offline (memorised + paper in a safe). Mirror onto
+   `mamba`.
+
+There is always exactly one irreducible offline root secret; here it is the
+Vaultwarden master password. Mirroring Vaultwarden onto `ubongo` is rejected: it
+would make the control node run a service (against its remit) and still need that
+master password.
+
+> verified: rbw offline-cache decryption · TO VERIFY before relying on the recovery
+> model · rbw docs · (ADR-014, security-relevant — confirm during build)
+
+## Consequences
+
+- The control node is physical compute outside the cluster, so it appears in
+  `docs/hardware/reference.md` even though it is not a cluster node (ADR-012).
+- All testing (Molecule, lint, staging/external) runs on `ubongo` (ADR-008).
+- A future **service-UI acceptance** testing level (Claude driving a headless browser
+  against a deployed service) is anticipated; `ubongo` is sized for it. The harness
+  is a separate spec.
+
+## Deferred (separate specs / discussions)
+
+1. **Mesh VPN choice** — Tailscale vs NetBird, hosted vs self-hosted. Recovery
+   dimension: a hosted coordinator keeps the mesh up when the cluster is down; a
+   self-hosted coordinator must live off-cluster (on `ubongo`), never on the fleet,
+   or it recreates the chicken-and-egg.
+2. **Browser-E2E verification harness** — Playwright/headless-Chromium, test-user
+   generation, screenshot-back-to-Claude, and the new ADR-008 level.
+3. **`rbw` offline-cache verification** — confirm offline decryption before relying
+   on it (ADR-014).
+
+## What was ruled out
+
+| Option | Reason |
+|---|---|
+| Keep control node as a cluster VM | Fails cold-start, recovery, always-on. |
+| Laptop-only (`mamba` for everything) | Fails always-on. Retained as break-glass backup. |
+| Split roles (control VM + thin jump box) | Two toolchains, split control plane, heavy testing back on a cluster VM. |
+| Mirror Vaultwarden onto `ubongo` | Control node would run a service; still needs the master password. |
+| Self-hosted mesh coordinator on the cluster | Recreates the chicken-and-egg. |
+| Raspberry Pi | Chokes running Docker + Chromium + toolchain together. |
+
+See also: ADR-001 (architecture), ADR-005 (bootstrapping), ADR-008 (testing),
+ADR-009 (provisioning handoff), ADR-012 (hardware/capacity), ADR-002 (security).