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-opensearch/README.md

@@ -0,0 +1,103 @@
+# wfe-opensearch
+
+OpenSearch search index provider for the WFE workflow engine.
+
+## What it does
+
+Implements the `SearchIndex` trait backed by OpenSearch. Indexes workflow instances as documents and supports full-text search with bool queries, term filters (status, reference), date range filters, and pagination. The index mapping is created automatically on `start()` if it does not already exist.
+
+## Quick start
+
+```rust
+use wfe_opensearch::OpenSearchIndex;
+
+let index = OpenSearchIndex::new(
+    "http://localhost:9200",
+    "wfe_workflows",
+)?;
+
+// Create the index mapping (idempotent)
+index.start().await?;
+
+// Index a workflow
+index.index_workflow(&workflow_instance).await?;
+
+// Search with filters
+use wfe_core::traits::search::{SearchFilter, SearchIndex};
+
+let page = index.search(
+    "deploy",         // free-text query
+    0,                // skip
+    20,               // take
+    &[SearchFilter::Status(WorkflowStatus::Complete)],
+).await?;
+```
+
+## API
+
+| Type | Trait |
+|------|-------|
+| `OpenSearchIndex` | `SearchIndex` |
+
+Key methods:
+
+| Method | Description |
+|--------|-------------|
+| `new(url, index_name)` | Create a provider pointing at an OpenSearch instance |
+| `start()` | Create the index with mappings if it does not exist |
+| `index_workflow(instance)` | Index or update a workflow document |
+| `search(terms, skip, take, filters)` | Bool query with filters, returns `Page<WorkflowSearchResult>` |
+| `client()` | Access the underlying `OpenSearch` client |
+| `index_name()` | Get the configured index name |
+
+### Search filters
+
+| Filter | OpenSearch mapping |
+|--------|--------------------|
+| `SearchFilter::Status(status)` | `term` query on `status` (keyword) |
+| `SearchFilter::Reference(ref)` | `term` query on `reference` (keyword) |
+| `SearchFilter::DateRange { field, before, after }` | `range` query on date fields |
+
+Free-text terms run a `multi_match` across `description`, `reference`, and `workflow_definition_id`.
+
+## Index mapping
+
+The index uses the following field types:
+
+| Field | Type |
+|-------|------|
+| `id` | keyword |
+| `workflow_definition_id` | keyword |
+| `version` | integer |
+| `status` | keyword |
+| `reference` | keyword |
+| `description` | text |
+| `data` | object (disabled -- stored but not indexed) |
+| `create_time` | date |
+| `complete_time` | date |
+
+## Configuration
+
+Constructor takes two arguments:
+
+| Parameter | Example | Description |
+|-----------|---------|-------------|
+| `url` | `http://localhost:9200` | OpenSearch server URL |
+| `index_name` | `wfe_workflows` | Index name for workflow documents |
+
+Security plugin is not required. For local development, run OpenSearch with `DISABLE_SECURITY_PLUGIN=true`.
+
+## Testing
+
+Requires a running OpenSearch instance. Use the project docker-compose:
+
+```sh
+docker compose up -d opensearch
+cargo test -p wfe-opensearch
+```
+
+Default test connection: `http://localhost:9200`
+
+## License
+
+MIT