.agents/skills: add mz-demo-data for live SQL-generated demo data

.agents/skills/README.md

@@ -45,3 +45,14 @@ choosing which framework to use, start with **mz-test**.
 | Skill | When to use | What it does |
 |---|---|---|
 | **mz-adapter-guide** | Working on or asking about the adapter layer | Correctness invariants and architectural notes for the coordinator, pgwire, peek paths, timestamp oracle, and related crates |
+
+## Demos
+
+| Skill | When to use | What it does |
+|---|---|---|
+| **mz-demo-data** | Standing up live, realistic data to show Materialize off | Builds continuously-updating synthetic data entirely in SQL, no Kafka or external load generator. Ships six domains (auctions, ecommerce, banking, IoT, clickstream, zoo) and a rubric for designing new ones |
+
+Unlike the skills above, `mz-demo-data` is aimed at people evaluating
+Materialize rather than developing it. Its assets live in `misc/demo-data/`,
+with `.agents/skills/mz-demo-data` as a symlink, so the directory can be
+lifted out on its own.

.agents/skills/mz-demo-data

@@ -0,0 +1 @@
+../../misc/demo-data

misc/demo-data/README.md

@@ -0,0 +1,50 @@
+# mz-demo-data
+
+Continuously-updating, realistic synthetic data for Materialize demos —
+generated entirely in SQL, no external load generator.
+
+Based on [this blog post](https://github.com/frankmcsherry/blog/blob/master/posts/2024-05-19.md).
+
+## Quickstart
+
+```sh
+PSQL="psql -p 6875 -h localhost -U materialize"
+
+$PSQL -f assets/scaffold.sql            # moments + random (24h window, 1s tick)
+$PSQL -f assets/common/people.sql       # shared 256-identity pool
+$PSQL -f assets/domains/auctions.sql    # or any other domain
+```
+
+Then in a psql session:
+
+```sql
+COPY (SUBSCRIBE (SELECT COUNT(*) FROM auctions) WITH (progress = true)) TO STDOUT;
+```
+
+To change the retention window or tick:
+
+```sql
+\set retention '6 hours'
+\set tick '1 second'
+\i assets/scaffold.sql
+```
+
+Teardown: `$PSQL -f assets/teardown.sql`.
+
+## Domains
+
+* **auctions** — marketplace; auctions with lifecycle and bids
+* **ecommerce** — orders, line items, totals (joins shared `people`)
+* **banking** — double-entry transactions; `SUM(balances) = 0` invariant
+* **iot** — devices, readings, threshold alerts
+* **clickstream** — sessions, page views, conversion funnel
+* **zoo** — zoo visits, ratings, shipments; four invariants at once
+
+Load more than one to see Materialize keep multiple data products in sync
+over the same shared identity space.
+
+## Designing a new domain
+
+See [`SKILL.md`](SKILL.md). The short version: derive PKs from `moment`'s
+random bytes, derive FKs by re-hashing the parent's bytes, control
+distributions with byte masks, and bake an invariant into the construction.