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-postgres/README.md

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+# wfe-postgres
+
+PostgreSQL persistence provider for the WFE workflow engine.
+
+## What it does
+
+Implements the full `PersistenceProvider` trait backed by PostgreSQL via sqlx. All workflow data, events, subscriptions, and scheduled commands live in a dedicated `wfc` schema. Uses JSONB for structured data (execution pointer children, scope, extension attributes) and TIMESTAMPTZ for timestamps. Schema and indexes are created automatically via `ensure_store_exists`.
+
+## Quick start
+
+```rust
+use wfe_postgres::PostgresPersistenceProvider;
+
+let provider = PostgresPersistenceProvider::new(
+    "postgres://wfe:wfe@localhost:5433/wfe_test"
+).await?;
+
+// Create schema and tables (idempotent)
+provider.ensure_store_exists().await?;
+```
+
+Wire it into the WFE host:
+
+```rust
+let host = WorkflowHost::new(provider);
+```
+
+## API
+
+| Type | Trait |
+|------|-------|
+| `PostgresPersistenceProvider` | `PersistenceProvider`, `WorkflowRepository`, `EventRepository`, `SubscriptionRepository`, `ScheduledCommandRepository` |
+
+Additional methods:
+
+- `truncate_all()` -- truncates all tables with CASCADE, useful for test cleanup
+
+## Configuration
+
+Connection string follows the standard PostgreSQL URI format:
+
+```
+postgres://user:password@host:port/database
+```
+
+The pool is configured with up to 10 connections. All tables are created under the `wfc` schema.
+
+## Schema
+
+Tables created in `wfc`:
+
+| Table | Purpose |
+|-------|---------|
+| `wfc.workflows` | Workflow instances. `data` is JSONB, timestamps are TIMESTAMPTZ. |
+| `wfc.execution_pointers` | Step state. `children`, `scope`, `extension_attributes` are JSONB. References `wfc.workflows(id)`. |
+| `wfc.events` | Published events. `event_data` is JSONB. |
+| `wfc.event_subscriptions` | Active subscriptions with CAS-style external token locking. |
+| `wfc.scheduled_commands` | Deferred commands. Unique on `(command_name, data)` with upsert semantics. |
+| `wfc.execution_errors` | Error log with auto-incrementing serial primary key. |
+
+Indexes are created on `next_execution`, `status`, `(event_name, event_key)`, `is_processed`, `event_time`, `workflow_id`, and `execute_time`.
+
+## Testing
+
+Requires a running PostgreSQL instance. Use the project docker-compose:
+
+```sh
+docker compose up -d postgres
+cargo test -p wfe-postgres
+```
+
+Default test connection string: `postgres://wfe:wfe@localhost:5433/wfe_test`
+
+## License
+
+MIT