Wire context event to preprocessor for per-LLM-call execution

- Change from contextTransform (runs once at agent start) to preprocessor
- preprocessor runs before EACH LLM call inside the agent loop
- ContextEvent now uses Message[] (pi-ai format) instead of AppMessage[]
- Deep copy handled by pi-ai preprocessor, not Agent

This enables:
- Pruning rules applied on every turn (not just agent start)
- /prune during long agent loop takes effect immediately
- Compaction can use same transforms (future work)

packages/coding-agent/src/core/hooks/runner.ts

@@ -2,7 +2,7 @@
  * Hook runner - executes hooks and manages their lifecycle.
  */
 
-import type { AppMessage } from "@mariozechner/pi-agent-core";
+import type { Message } from "@mariozechner/pi-ai";
 import type { ModelRegistry } from "../model-registry.js";
 import type { SessionManager } from "../session-manager.js";
 import type { AppendEntryHandler, LoadedHook, SendMessageHandler } from "./loader.js";
@@ -311,12 +311,13 @@ export class HookRunner {
 	/**
 	 * Emit a context event to all hooks.
 	 * Handlers are chained - each gets the previous handler's output (if any).
-	 * Returns the final modified messages, or undefined if no modifications.
+	 * Returns the final modified messages, or the original if no modifications.
+	 *
+	 * Note: Messages are already deep-copied by the caller (pi-ai preprocessor).
 	 */
-	async emitContext(messages: AppMessage[]): Promise<AppMessage[] | undefined> {
+	async emitContext(messages: Message[]): Promise<Message[]> {
 		const ctx = this.createContext();
 		let currentMessages = messages;
-		let modified = false;
 
 		for (const hook of this.hooks) {
 			const handlers = hook.handlers.get("context");
@@ -331,7 +332,6 @@ export class HookRunner {
 
 					if (handlerResult && (handlerResult as ContextEventResult).messages) {
 						currentMessages = (handlerResult as ContextEventResult).messages!;
-						modified = true;
 					}
 				} catch (err) {
 					const message = err instanceof Error ? err.message : String(err);
@@ -344,6 +344,6 @@ export class HookRunner {
 			}
 		}
 
-		return modified ? currentMessages : undefined;
+		return currentMessages;
 	}
 }

packages/coding-agent/src/core/hooks/types.ts

@@ -6,7 +6,7 @@
  */
 
 import type { AppMessage } from "@mariozechner/pi-agent-core";
-import type { ImageContent, Model, TextContent, ToolResultMessage } from "@mariozechner/pi-ai";
+import type { ImageContent, Message, Model, TextContent, ToolResultMessage } from "@mariozechner/pi-ai";
 import type { Component } from "@mariozechner/pi-tui";
 import type { Theme } from "../../modes/interactive/theme/theme.js";
 import type { CompactionPreparation, CompactionResult } from "../compaction.js";
@@ -148,13 +148,14 @@ export type SessionEvent =
 
 /**
  * Event data for context event.
- * Fired before messages are sent to the LLM, allowing hooks to modify context non-destructively.
+ * Fired before each LLM call, allowing hooks to modify context non-destructively.
  * Original session messages are NOT modified - only the messages sent to the LLM are affected.
+ * Messages are already in LLM format (Message[], not AppMessage[]).
  */
 export interface ContextEvent {
 	type: "context";
-	/** Messages about to be sent to the LLM */
-	messages: AppMessage[];
+	/** Messages about to be sent to the LLM (deep copy, safe to modify) */
+	messages: Message[];
 }
 
 /**
@@ -330,7 +331,7 @@ export type HookEvent =
  */
 export interface ContextEventResult {
 	/** Modified messages to send instead of the original */
-	messages?: AppMessage[];
+	messages?: Message[];
 }
 
 /**

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

@@ -589,7 +589,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 			tools: allToolsArray,
 		},
 		messageTransformer,
-		contextTransform: hookRunner
+		preprocessor: hookRunner
 			? async (messages) => {
 					return hookRunner.emitContext(messages);
 				}