memory-neo4j: implement mid-session core memory refresh

Add `coreMemory.refreshAtContextPercent` config option to re-inject
core memories when context usage exceeds a threshold. This counters
the "lost in the middle" phenomenon documented by Liu et al. (2023).

Implementation:
- Extend before_agent_start hook event with context usage info
- Pass contextWindowTokens and estimatedUsedTokens to hooks
- Track mid-session refresh per session to prevent over-refreshing
- Clear refresh tracking on compaction
- Add comprehensive tests

Based on research: Liu et al., "Lost in the Middle: How Language
Models Use Long Contexts" (Stanford, 2023)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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

@@ -31,6 +31,8 @@ import {
   listChannelSupportedActions,
   resolveChannelMessageToolHints,
 } from "../../channel-tools.js";
+import { estimateMessagesTokens } from "../../compaction.js";
+import { DEFAULT_CONTEXT_TOKENS } from "../../defaults.js";
 import { resolveOpenClawDocsPath } from "../../docs-path.js";
 import { isTimeoutError } from "../../failover-error.js";
 import { resolveModelAuthMode } from "../../model-auth.js";
@@ -850,10 +852,16 @@ export async function runEmbeddedAttempt(
         let effectivePrompt = params.prompt;
         if (hookRunner?.hasHooks("before_agent_start")) {
           try {
+            // Calculate context usage for mid-session memory refresh
+            const contextWindowTokens = params.model.contextWindow ?? DEFAULT_CONTEXT_TOKENS;
+            const estimatedUsedTokens = estimateMessagesTokens(activeSession.messages);
+
             const hookResult = await hookRunner.runBeforeAgentStart(
               {
                 prompt: params.prompt,
                 messages: activeSession.messages,
+                contextWindowTokens,
+                estimatedUsedTokens,
               },
               {
                 agentId: hookAgentId,

src/plugins/types.ts

@@ -343,6 +343,10 @@ export type PluginHookBootstrapResult = {
 export type PluginHookBeforeAgentStartEvent = {
   prompt: string;
   messages?: unknown[];
+  /** Model's total context window in tokens. */
+  contextWindowTokens?: number;
+  /** Estimated tokens currently used in context. */
+  estimatedUsedTokens?: number;
 };
 
 export type PluginHookBeforeAgentStartResult = {