docs(prd-0070): resolve build-placement open question — builds stay in the orchestrator

v1 decision: agent-image builds run in the orchestrator (the control-plane
side already owns launches and hosts builds in today's combined unit; the
data-plane gateway must not build). Consequence recorded in the memory
section: the orchestrator keeps the ~4 GB buildah ceiling and the gateway
is the slim unit (~1 GB), so the split is ~memory-neutral-to-+1 GB for now
— the ~1.5 GB win waits on a later move to a dedicated slim build unit
(#468-adjacent). Also flags the tension of co-locating buildah with the
control plane. Removed the corresponding open question.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/prds/0070-per-host-orchestrator.md

@@ -427,27 +427,32 @@ unchanged. A *compromised gateway* can already reach the orchestrator today
 the split adds **no new surface** for the gateway-compromise case; it only
 closes the *agent* path on Docker/macOS.
 
-#### Memory: right-size, don't double
+#### Memory: fixed ceilings, and where builds run
 
 Firecracker guest memory is a **fixed ceiling** set at boot (`mem_size_mib`);
 it's demand-paged by the host (RSS grows with what the guest touches) but there
 is **no reclaim without a balloon device**, and none is configured — so plan
 for the ceiling. The combined infra VM is **4096 MiB**, but that headroom is
 for **buildah** (in-guest agent-image builds), not the daemons, which are
-light.
+light. Steady-state daemon footprints are small: the **orchestrator**
+(stdlib-only Python HTTP + SQLite) is a few hundred MiB; the **gateway**
+(mitmproxy TLS bump + DLP body buffering, git-http, supervise, gitleaks) is
+**512 MiB–1 GB**.
 
-Split naïvely that reads as 2×, but right-sized it is **less** than today:
-
-- **Orchestrator** — stdlib-only Python HTTP + SQLite, no builds → **256–512
-  MiB**.
-- **Gateway** — mitmproxy (TLS bump + DLP body buffering) + git-http +
-  supervise + gitleaks → **512 MiB–1 GB**, *if it does not build images*.
-
-The real lever is **where agent-image builds run** (buildah's 2–4 GB working
-set). Move builds off the gateway — to a host-controller / dedicated build unit
-(a #468-adjacent decision) — and a slim orchestrator (~512 MiB) + slim gateway
-(~1 GB) ≈ **1.5 GB total vs. today's 4 GB**. A per-unit **balloon device** to
-reclaim idle memory is a later option, not needed for v1.
+The real ceiling driver is **where agent-image builds run** (buildah's 2–4 GB
+working set). **v1 decision: builds stay in the orchestrator** — it is the
+control-plane/management side that owns launches (and already hosts builds in
+today's combined unit), and the data-plane gateway must not build. So the
+**orchestrator keeps a ~4 GB ceiling** (mostly idle, spiking during a build)
+and the **gateway is the slim unit** (~1 GB). Net for v1 the split is therefore
+roughly **memory-neutral to ~+1 GB** — a slim gateway added beside a
+still-build-capable orchestrator — *not* the ~1.5 GB total; that win is
+deferred to a later move of builds onto a **dedicated slim build unit**
+(#468-adjacent). Co-locating buildah with the control plane also keeps a
+network-heavy, image-defined workload next to the signing key + registry — a
+known tension the dedicated build unit resolves later. A per-unit **balloon
+device** to reclaim the orchestrator's build-vs-idle swing is the other future
+lever. The access and surface-separation gains land now regardless.
 
 #### iroh belongs in the orchestrator
 
@@ -550,11 +555,6 @@ Keep the gateway **service one shared thing** throughout.
   plane (add/remove routes/keys/proposals without a restart).
 - **Identity-token delivery:** exactly how the per-bottle token is placed
   where the agent can present it but not swap in another bottle's.
-- **Where agent-image builds run once the planes split:** buildah's 2–4 GB
-  working set is what keeps the combined infra VM at 4 GB. Keeping the gateway
-  slim (~1 GB) means builds move off it — to a host-controller / dedicated
-  build unit (#468-adjacent) — or the gateway keeps the build headroom and the
-  memory win is smaller. Decides the gateway VM/container size.
 - **Agent-PTY-to-mobile streaming path:** relay it through the orchestrator's
   iroh endpoint (default — one authenticated remote door, plane split intact),
   or give the data-plane stream its own transport. Only matters once remote