docs: README + plan/ADR for slice 5 (Neo4j GraphBackend)

Updates the POC README with a 'Storage backends' section
documenting the GraphBackend Protocol, the two implementations
(InMemoryGraph and Neo4jGraph), and the LORE_GRAPH_BACKEND
env-var selection in the MCP entry scripts. Adds the slice 5
plan doc (docs/plan/05-slice-neo4j-backend.md in the design
repo) and ADR 0011 capturing the Protocol + dual-write
model decisions.

README.md

@@ -35,6 +35,61 @@ The 44 other MCP tools, the consistency engine, the TypeTemplate
 polymorphic extension, the plane model, the MCP server wiring. All
 deferred to follow-up slices per the design.
 
+## Storage backends (slice 5)
+
+The 36 MCP tools and the consistency engine read from a
+`GraphBackend` Protocol (`lore_engine_poc/graph_backend.py`)
+with two implementations:
+
+* **InMemoryGraph** (default, pickle-backed). The in-memory
+  dict-of-dicts from slices 1-11. Used for fast tests, the
+  CI test suite, and the Docker image's baked-in
+  `.graph.pkl` (slice 11).
+* **Neo4jGraph** (slice 5.3+). A `neo4j:5` container with
+  the reified `:Relation` shape (ADR 0009). The production
+  graph substrate per ADR 0008.
+
+The MCP entry scripts (`scripts/05_mcp_server.py`,
+`scripts/06_mcp_http_server.py`) select the backend at
+startup via `LORE_GRAPH_BACKEND`:
+
+```bash
+# Pickle (default) — load .graph.pkl
+python3 scripts/05_mcp_server.py
+# or
+python3 scripts/06_mcp_http_server.py
+
+# Neo4j — connect to bolt://$LORE_NEO4J_URI
+LORE_GRAPH_BACKEND=neo4j LORE_NEO4J_URI=bolt://localhost:7687 \
+  python3 scripts/06_mcp_http_server.py
+```
+
+Compose brings up the Neo4j stack with a profile:
+
+```bash
+# Pickle-backed MCP (slice 11 default)
+docker compose --profile pickle up -d
+
+# Neo4j-backed MCP (production substrate, slice 5)
+docker compose --profile neo4j up -d
+```
+
+The Neo4j compose stack includes:
+
+* `neo4j:5` — the database (`NEO4J_AUTH=none` for loopback;
+  **switch to a username/password before any non-loopback
+  exposure**).
+* `lore-engine-ingest` — one-shot job that runs
+  `01_ingest.py --write-neo4j --skip-cognee` after Neo4j
+  is healthy.
+* `lore-engine-mcp-neo4j` — the MCP HTTP server reading
+  from Neo4j (depends on both `neo4j` healthy and
+  `lore-engine-ingest` exited 0).
+
+The pickle profile keeps working exactly as in slice 11.4
+(pickup unchanged, MCP server on port 8765, baked
+`.graph.pkl`).
+
 ## Run
 
 ```bash