chore: baseline container POC and atomic foundation plan

- Containerized Pi hello-world proof (image mosaic-poc-agent:0.84.4, non-root)
- Four immutable contract fixtures loaded into a generated system prompt
- build/hello/verify/reset scripts with exact-match gating and reset safety
- Documented Pi discovery (v0.84.4, -p mode, --system-prompt, container auth)
- Append-only BUILD-LOG with corrections; deferred layers in LAYERS.md
- Architecture plan: docs/plans/2026-09-02_atomic-mosaic-foundation.md

LAYERS.md

@@ -0,0 +1,50 @@
+# LAYERS
+
+Deferred capability layers for the Mosaic experiment. Only L0 is implemented by
+this proof of concept; everything below it is documented here and deliberately
+not implemented (see BRIEF.md, "Explicit exclusions").
+
+## L0 — Implemented: container returns MOSAIC_HELLO_OK
+
+One image (`mosaic-poc-agent:0.84.4`, built on `node:24-bookworm-slim`, non-root,
+pinned Pi) runs one Pi agent one-shot. Four immutable local contract files are
+loaded in fixed order into the generated system prompt
+(`/var/lib/mosaic/system-prompt.md`). One real model request is sent
+noninteractively; the response must equal `MOSAIC_HELLO_OK` exactly or the
+verification exits nonzero. Authentication is supplied at runtime only
+(read-only mounted pi auth file, or a provider API key environment variable).
+
+## L1 — Deferred: persist and resume a named Pi session
+
+Keep a named Pi session across container runs (`--name`, session storage under
+`/var/lib/mosaic`), resume it with the documented session flags, and verify
+state survives a container restart.
+
+## L2 — Deferred: fixed tool permission policy
+
+Add a fixed allow/deny policy for Pi tools (e.g. restricting built-in tools via
+documented `--tools` / `--exclude-tools` or an extension-based permission gate),
+so contract files can constrain what the agent may do, not just what it says.
+
+## L3 — Deferred: load full versioned contract bundles
+
+Replace the four static fixtures with versioned contract bundles: bundle
+manifests, contract versions, and deterministic ordering/hashing, loaded from
+an immutable bundle artifact instead of files copied at image build time.
+
+## L4 — Deferred: Claude as a second runtime
+
+Add a second runtime (Claude) alongside the Pi agent in the same container
+stack, behind the same contract-loading path, to compare behavior across
+runtimes.
+
+## L5 — Deferred: multiple agents and communication
+
+Run several named agents with defined roles and a communication channel between
+them (message passing or shared state under `/var/lib/mosaic`).
+
+## L6 — Deferred: orchestration, knowledge storage, and portal features
+
+Fleet-level orchestration, knowledge storage, monitoring, and portal UI on top
+of L1-L5. This is where the existing Mosaic Stack concepts would be re-evaluated
+from first principles.