feat(sources)!: split source contracts into PollSource/StreamSource and add mode-aware source config

- Introduce explicit source interfaces: sources.PollSource and sources.StreamSource, with shared sources.Input (Name() only).
- Remove mandatory Kind() from the base source contract to support sources that emit multiple kinds.
- Add config.SourceMode (poll, stream, or omitted/auto) and SourceConfig.Kinds (plural expected kinds), while keeping legacy SourceConfig.Kind for compatibility.
- Enforce mode semantics in config validation (poll requires every, stream forbids every) and detect mode/driver mismatches in sources.Registry.
- Update docs and tests for the new source model and config behavior.

sources/doc.go

@@ -0,0 +1,14 @@
+// Package sources defines feedkit's input-source abstraction.
+//
+// A source ingests upstream input and emits one or more event.Event values.
+//
+// feedkit supports two source modes:
+//   - PollSource: scheduler invokes Poll on a cadence.
+//   - StreamSource: source runs continuously and pushes events as input arrives.
+//
+// Source drivers are domain-specific and registered into Registry by driver name.
+// Registry can then build configured sources from config.SourceConfig.
+//
+// A single source may emit 0..N events per poll or stream iteration, and those
+// events may span multiple event kinds.
+package sources