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/rpc/rpc-mode.ts

@@ -348,6 +348,7 @@ export async function runRpcMode(session: AgentSession): Promise<never> {
 					.prompt(command.message, {
 						images: command.images,
 						streamingBehavior: command.streamingBehavior,
+						source: "rpc",
 					})
 					.catch((e) => output(error(id, "prompt", e.message)));
 				return success(id, "prompt");