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/modes/print-mode.ts

@@ -99,7 +99,7 @@ export async function runPrintMode(session: AgentSession, options: PrintModeOpti
 					return { cancelled: result.cancelled };
 				},
 			},
-			// No UI context
+			// No UI context - hasUI will be false
 		);
 		extensionRunner.onError((err) => {
 			console.error(`Extension error (${err.extensionPath}): ${err.error}`);