feat(coding-agent): add input event for extension input interception (#761)

* feat(coding-agent): add input event for extension input interception

Extensions can now intercept, transform, or handle user input before the
agent processes it. Three result types: continue (pass through), transform
(modify text/images), handled (respond without LLM). Handlers chain
transforms and short-circuit on handled. Source field identifies origin.

* fix: make source public, use if/else over ternary

* fix: remove response field, extension handles own UI

packages/coding-agent/src/index.ts

@@ -64,6 +64,9 @@ export type {
 	ExtensionShortcut,
 	ExtensionUIContext,
 	ExtensionUIDialogOptions,
+	InputEvent,
+	InputEventResult,
+	InputSource,
 	KeybindingsManager,
 	LoadExtensionsResult,
 	MessageRenderer,