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/docs/session-tree-plan.md

@@ -227,7 +227,7 @@ interface HookUIContext {
 
 See also: `CustomEntry<T>` for storing hook state that does NOT participate in context.
 
-**New: `context` event (TODO)**
+**New: `context` event ✅**
 
 Fires before messages are sent to the LLM, allowing hooks to modify context non-destructively.