docs: document custom type support throughout project

Update all documentation to reflect that custom types work automatically
with the #[task] macro, requiring only Clone + Send + Sync + 'static.

Documentation updates:
- README: Add custom types to features, new Custom Types section
- CHANGELOG: Document custom type improvement in [Unreleased]
- src/lib.rs: Add custom types example to main API docs
- src/task.rs: Update Task trait docs to mention macro-generated extraction
- src/extract.rs: Clarify ExtractInput is now internal-only (for task_fn)
- dagx-macros: Update macro docs to emphasize automatic type handling

New example:
- examples/custom_types.rs: Simple 120-line example showing User type
  flowing through DAG with fan-out pattern (one output → two consumers)
  demonstrating Arc wrapping is automatic and transparent

This makes the custom type capability discoverable and prominent for users,
turning a previous limitation into a headline feature.

src/task.rs

@@ -8,7 +8,11 @@ use std::future::Future;
 /// A unit of async work with typed inputs and outputs.
 ///
 /// Use the [`task`](crate::task) macro to implement this trait. The macro automatically
-/// derives `Input` and `Output` types from your `run()` method signature.
+/// derives `Input` and `Output` types from your `run()` method signature and generates
+/// type-specific extraction logic in `extract_and_run()`.
+///
+/// **Custom types work automatically!** As long as your output type implements
+/// `Clone + Send + Sync + 'static`, the macro handles everything. No trait implementations needed!
 ///
 /// # Task Patterns
 ///