Add context event for non-destructive message modification before LLM calls

- Add contextTransform option to Agent (runs before messageTransformer)
- Deep copy messages before passing to contextTransform (modifications are ephemeral)
- Add ContextEvent and ContextEventResult types
- Add emitContext() to HookRunner (chains multiple handlers)
- Wire up in sdk.ts when creating Agent with hooks

Enables dynamic context pruning: hooks can modify messages sent to LLM
without changing session data. See discussion #330.

packages/coding-agent/src/core/sdk.ts

@@ -589,6 +589,11 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 			tools: allToolsArray,
 		},
 		messageTransformer,
+		contextTransform: hookRunner
+			? async (messages) => {
+					return hookRunner.emitContext(messages);
+				}
+			: undefined,
 		queueMode: settingsManager.getQueueMode(),
 		transport: new ProviderTransport({
 			getApiKey: async () => {