fix: remove systemPrompt from llm_input result type (not yet plumbed)

Address greptile review: systemPrompt was declared in PluginHookLlmInputResult
and merged in the hook runner, but never applied in attempt.ts. Since the
system prompt is finalised before the llm_input hook runs, applying it
requires restructuring that's out of scope for this PR.

Removes the misleading field to avoid a silent no-op for plugin authors.
Will be re-added when late-stage system prompt overrides are supported.

src/agents/pi-embedded-runner/run/attempt.ts

@@ -1015,7 +1015,7 @@ export async function runEmbeddedAttempt(
             );
           }
 
-          // Run llm_input hook — plugins may modify prompt/systemPrompt
+          // Run llm_input hook — plugins may modify the user prompt
           if (hookRunner?.hasHooks("llm_input")) {
             try {
               const llmInputResult = await hookRunner.runLlmInput(

src/plugins/hooks.ts

@@ -283,7 +283,7 @@ export function createHookRunner(registry: PluginRegistry, options: HookRunnerOp
   /**
    * Run llm_input hook.
    * Allows plugins to observe or modify the input payload sent to the LLM.
-   * Plugins can return `{ prompt, systemPrompt }` to transform the input,
+   * Plugins can return `{ prompt }` to transform the input,
    * or return void/undefined for observation-only (backward compatible).
    */
   async function runLlmInput(
@@ -296,7 +296,6 @@ export function createHookRunner(registry: PluginRegistry, options: HookRunnerOp
       ctx,
       (acc, next) => ({
         prompt: next.prompt ?? acc?.prompt,
-        systemPrompt: next.systemPrompt ?? acc?.systemPrompt,
       }),
     );
   }

src/plugins/types.ts

@@ -377,8 +377,9 @@ export type PluginHookLlmInputEvent = {
 export type PluginHookLlmInputResult = {
   /** Modified prompt text. If set, replaces the original prompt. */
   prompt?: string;
-  /** Modified system prompt. If set, replaces the original system prompt. */
-  systemPrompt?: string;
+  // Note: systemPrompt modification is not yet supported — the system prompt
+  // is finalised earlier in the pipeline. Will be added when late-stage
+  // system prompt overrides are plumbed through.
 };
 
 // llm_output hook