fix: migrate auto-recall from before_agent_start to before_prompt_build (#1312)

## Problem

`before_agent_start` is a legacy hook that the OpenClaw framework
invokes **twice** per agent run:

1. **Model resolve phase** ([`run.ts`
L348](https://github.com/openclaw/openclaw/blob/main/src/agents/pi-embedded-runner/run.ts#L348)):
passes only `{ prompt }` — no `messages` field
2. **Prompt build phase** ([`attempt.ts`
L1438](https://github.com/openclaw/openclaw/blob/main/src/agents/pi-embedded-runner/run/attempt.ts#L1438)):
passes `{ prompt, messages }` — messages available

This causes the auto-recall handler to fire twice: the first invocation
has no `messages` so topic-aware pre-filtering (if any) is bypassed, and
a redundant embedding search + LLM filter call runs on every turn.

## Solution

Use `before_prompt_build` instead
([docs](https://docs.openclaw.ai/tools/plugin#plugin-hooks)), which:
- Fires once per turn, after session load
- Always provides `{ prompt, messages }` — reliable access to
conversation history
- Is the framework-recommended hook for prompt injection (per OpenClaw
docs: *"legacy compatibility hook... prefer the explicit hooks above"*)
- Returns the same result fields: `prependContext`, `systemPrompt`,
`prependSystemContext`, `appendSystemContext`

## Changes

- `before_agent_start` → `before_prompt_build` (single line change in
`index.ts` L931)
- No other logic changes; return contract is identical

## References

- OpenClaw agent loop:
[`concepts/agent-loop.md`](https://github.com/openclaw/openclaw/blob/main/docs/concepts/agent-loop.md)
- OpenClaw plugin docs:
[`tools/plugin.md`](https://github.com/openclaw/openclaw/blob/main/docs/tools/plugin.md)
- Related issue:
[openclaw/openclaw#26914](openclaw/openclaw#26914)
(ephemeralContext request — same root cause of prependContext
accumulation)

Author: syzsunshine219

apps/memos-local-openclaw/index.ts

@@ -1806,7 +1806,7 @@ Groups: ${groupNames.length > 0 ? groupNames.join(", ") : "(none)"}`,
 
     // ─── Auto-recall: inject relevant memories before agent starts ───
 
-    api.on("before_agent_start", async (event: { prompt?: string; messages?: unknown[] }, hookCtx?: { agentId?: string; sessionKey?: string }) => {
+    api.on("before_prompt_build", async (event: { prompt?: string; messages?: unknown[] }, hookCtx?: { agentId?: string; sessionKey?: string }) => {
       if (!allowPromptInjection) return {};
       if (!event.prompt || event.prompt.length < 3) return;