coding-agents (Pi): beforeAgentStart replaces the whole system prompt, so other extensions' prompt sections are dropped #4841

Bug Description

In Pi, createPiHooks().beforeAgentStart returns { systemPrompt: event.systemPrompt + injection }. Pi treats a returned systemPrompt as a full replacement of the prompt for that run, and from then on it ignores event.systemPromptOptions.sections. So a section added by any extension whose before_agent_start handler runs after Hindsight's is dropped, on every turn where Hindsight injects memory.

I hit this with two mode extensions that add a prompt section while the mode is on. With Hindsight loaded, the modes showed as on, but their text never reached the model. With HINDSIGHT_DISABLE_HOOKS=1 it came back.

Code:

Steps to Reproduce

No Hindsight server needed. Two stub extensions:

// force.ts: what the Hindsight Pi hook does
export default function (pi) {
  pi.on("before_agent_start", (event) => ({ systemPrompt: `${event.systemPrompt}\n\n<memory>turn 1</memory>` }));
}

// section.ts: any extension that adds a prompt section
export default function (pi) {
  pi.on("before_agent_start", (event) => {
    event.systemPromptOptions.sections.other_ext = "SECTION-MARKER";
  });
}

1. pi --no-extensions -e ./force.ts -e ./section.ts -p "Reply OK", with a third extension that writes the before_provider_request payload to a file.
2. Look for SECTION-MARKER in the system message.
3. Run again with the order swapped: -e ./section.ts -e ./force.ts.

Expected Behavior

Memory and SECTION-MARKER both reach the model, whatever the load order.

Actual Behavior

• force.ts first: memory present, SECTION-MARKER missing.
• section.ts first: both present.

Suggested fix

Add the injection as a section and keep the old path for hosts without systemPromptOptions:

if (event.systemPromptOptions?.sections) {
  event.systemPromptOptions.sections.hindsight = injection;
  return undefined;
}
return { systemPrompt: `${event.systemPrompt}\n\n${injection}` };

I checked this with the same stubs: memory and SECTION-MARKER both arrive, in either order. Pi's extension docs say the same: "Returning systemPrompt, or setting forceSystemPrompt, replaces the whole prompt for that run."

Version

@vectorize-io/hindsight-coding-agents 0.7.0 (same code on main at ccfe85b), Pi 0.87.1, macOS.