diff --git a/README.md b/README.md
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+++ b/README.md
@@ -162,3 +162,15 @@ See [`docs/TRAINING_FLOW_V2.md`](docs/TRAINING_FLOW_V2.md) for full plan.
 ## License
 
 MIT
+
+---
+
+## Repo boundary
+
+This is the **model plane** of the IGLA marathon. The control plane (Railway
+client, `tri-railway` CLI, `tri-gardener` autonomous orchestrator,
+`gardener_runs` ledger) lives in [`gHashTag/trios-railway`](https://github.com/gHashTag/trios-railway).
+See [`docs/adr/0001-repo-boundaries.md`](docs/adr/0001-repo-boundaries.md) for
+the binding contract.
+
+`phi^2 + phi^-2 = 3 · TRINITY · NEVER STOP`
diff --git a/docs/adr/0001-repo-boundaries.md b/docs/adr/0001-repo-boundaries.md
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@@ -0,0 +1,75 @@
+# ADR-0001 — Repo boundaries: `trios-railway` vs `trios-trainer-igla`
+
+- **Status:** Accepted
+- **Date:** 2026-04-28
+- **Authors:** gHashTag (operator), tri-gardener team
+- **Anchor:** `phi^2 + phi^-2 = 3 · TRINITY · NEVER STOP`
+
+## Context
+
+The IGLA marathon (Gate-2 BPB < 1.85, Gate-3 BPB < 1.5) ships through two
+independent repositories. Earlier in the race the boundary between them
+was implicit, which led to a near-miss: an operator considered deleting
+`trios-trainer-igla` while reasoning about the gardener layout. This
+ADR makes the boundary explicit so it survives sleep, shift handoff, and
+LLM context loss.
+
+## Decision
+
+There are exactly two repositories on the IGLA critical path. Each has
+one job. They never depend on each other's source — only on shipped
+artefacts (Docker images, GHCR tags, JSONL ledgers).
+
+### `gHashTag/trios-railway` — control plane
+
+- Owns the typed Railway GraphQL client (`crates/trios-railway-core`).
+- Owns `bin/tri-railway` (operator CLI: `plan9`, `service deploy`, etc.).
+- Owns `bin/tri-gardener` (autonomous orchestrator, ASHA rungs,
+  plateau detector, queue picker, `gardener_runs` ledger).
+- Owns the Neon `railway_audit_*` and `gardener_runs` schemas.
+- Never contains trainer model code, training scripts, or weights.
+- May depend on the **artefact** `ghcr.io/ghashtag/trios-trainer-igla:*`
+  by tag/digest only.
+
+### `gHashTag/trios-trainer-igla` — model plane
+
+- Owns the trainer (`crates/trios-trainer-igla`, attention/JEPA/etc.).
+- Owns `Dockerfile` that produces `ghcr.io/ghashtag/trios-trainer-igla`.
+- Owns `assertions/seed_results.jsonl` and BPB-emitting code paths.
+- Never imports `trios-railway-core` or any control-plane crate.
+- May read its own deploy environment variables but does not call
+  Railway GraphQL itself — that is the control plane's job.
+
+### Bidirectional contract
+
+- The trainer image's environment is set by the control plane via
+  `variableUpsert`. Variable names are part of the contract and
+  versioned in `docs/contracts/trainer-env.md` (control plane).
+- BPB telemetry flows trainer → Neon `bpb_samples` (writer in trainer)
+  and Neon `bpb_samples` → gardener (reader in control plane). The
+  schema is owned by the control plane.
+
+## Consequences
+
+- The trainer repo is **NEVER** deleted, archived, or renamed as part
+  of any control-plane refactor. If a control-plane task description
+  mentions touching the trainer repo for anything other than the
+  trainer's own code or this ADR, the operator stops and re-asks.
+- Control-plane crates do not import trainer code. PRs that try are
+  rejected.
+- Trainer crates do not import `trios-railway-core`. PRs that try are
+  rejected.
+- A change to the trainer image schema (env vars, ports, ledger paths)
+  requires a paired commit: contract update in `trios-railway` and
+  the actual change in `trios-trainer-igla`, both referenced in the
+  PR body.
+
+## References
+
+- Tracker: [`gHashTag/trios-railway#43`](https://github.com/gHashTag/trios-railway/issues/43)
+- Gardener spec: [`gHashTag/trios-railway#49`](https://github.com/gHashTag/trios-railway/issues/49)
+- Gardener PR-1: [`gHashTag/trios-railway#50`](https://github.com/gHashTag/trios-railway/pull/50)
+- Race: [`gHashTag/trios#143`](https://github.com/gHashTag/trios/issues/143)
+- φ-physics foundation: [`gHashTag/trios#329`](https://github.com/gHashTag/trios/pull/329)
+
+`phi^2 + phi^-2 = 3 · TRINITY · NEVER STOP`