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/hooks/index.ts

@@ -13,6 +13,8 @@ export type {
 	AgentEndEvent,
 	AgentStartEvent,
 	BashToolResultEvent,
+	ContextEvent,
+	ContextEventResult,
 	CustomToolResultEvent,
 	EditToolResultEvent,
 	ExecOptions,