docs: add README for workspace and all 7 crates

Root README covers architecture, Rust builder API quick start,
YAML pipeline quick start, provider table, deno executor overview,
feature flags, and testing instructions.

Per-crate READMEs follow consistent structure: one-liner, what it
does, quick start code example, API reference, configuration,
testing, license. Engineering-confident tone throughout.

wfe/README.md

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+# wfe
+
+The umbrella crate for the WFE workflow engine.
+
+## What it does
+
+`wfe` provides `WorkflowHost`, the main orchestrator that ties persistence, locking, queuing, and step execution into a single runtime. It re-exports everything from `wfe-core` so downstream code only needs one dependency. Register workflow definitions, start instances, publish events, suspend/resume/terminate -- all through `WorkflowHost`.
+
+## Quick start
+
+```rust
+use std::sync::Arc;
+use wfe::{WorkflowHostBuilder, WorkflowHost};
+
+// Build a host with your providers.
+let host = WorkflowHostBuilder::new()
+    .use_persistence(persistence)
+    .use_lock_provider(lock_provider)
+    .use_queue_provider(queue_provider)
+    .build()
+    .unwrap();
+
+// Register a workflow definition.
+host.register_workflow::<serde_json::Value>(
+    &|builder| {
+        builder
+            .start_with::<MyStep>()
+            .name("do the thing")
+            .end_workflow()
+    },
+    "my-workflow",
+    1,
+).await;
+
+// Register custom steps.
+host.register_step::<MyStep>().await;
+
+// Start the background consumers.
+host.start().await.unwrap();
+
+// Launch a workflow instance.
+let id = host.start_workflow("my-workflow", 1, serde_json::json!({})).await.unwrap();
+
+// Publish an event to resume a waiting workflow.
+host.publish_event("approval", "order-123", serde_json::json!({"approved": true})).await.unwrap();
+```
+
+### Synchronous runner (testing)
+
+`run_workflow_sync` starts a workflow and polls until it completes or times out. Useful in integration tests.
+
+```rust
+use std::time::Duration;
+use wfe::run_workflow_sync;
+
+let result = run_workflow_sync(&host, "my-workflow", 1, data, Duration::from_secs(5)).await?;
+assert_eq!(result.status, wfe::models::WorkflowStatus::Complete);
+```
+
+## API
+
+| Type | Description |
+|---|---|
+| `WorkflowHost` | Main orchestrator. Registers workflows/steps, starts instances, publishes events, manages lifecycle. |
+| `WorkflowHostBuilder` | Fluent builder for `WorkflowHost`. Requires persistence, lock provider, and queue provider. Optionally accepts lifecycle publisher and search index. |
+| `InMemoryWorkflowRegistry` | In-memory `WorkflowRegistry` implementation. Stores definitions keyed by `(id, version)`. |
+| `run_workflow_sync` | Async helper that starts a workflow and polls until completion or timeout. |
+| `purge_workflows` | Cleanup utility for removing completed workflow data from persistence. |
+
+Everything from `wfe-core` is re-exported: `WorkflowBuilder`, `StepBody`, `ExecutionResult`, models, traits, and primitives.
+
+## Features
+
+| Feature | Description |
+|---|---|
+| `otel` | Enables OpenTelemetry tracing. Pulls in `opentelemetry`, `opentelemetry_sdk`, `opentelemetry-otlp`, and `tracing-opentelemetry`. Also enables `wfe-core/otel`. |
+
+## Testing
+
+```sh
+cargo test -p wfe
+```
+
+Tests use `wfe-sqlite` and `wfe-core/test-support` for in-memory persistence. No external services required.
+
+## License
+
+MIT