feat(preemptive-compaction): add onBeforeSummarize callback and context injection

- Added BeforeSummarizeCallback type to allow injecting context before session summarization
- Added onBeforeSummarize option to PreemptiveCompactionOptions
- Created compaction-context-injector module that injects summarization instructions with sections:
  - User Requests (As-Is)
  - Final Goal
  - Work Completed
  - Remaining Tasks
  - MUST NOT Do (Critical Constraints)
- Wired up callback invocation in preemptive-compaction before calling summarize API
- Exported new hook from src/hooks/index.ts

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src/hooks/compaction-context-injector/index.ts

@@ -0,0 +1,53 @@
+import type { SummarizeContext } from "../preemptive-compaction"
+import { injectHookMessage } from "../../features/hook-message-injector"
+import { log } from "../../shared/logger"
+
+const SUMMARIZE_CONTEXT_PROMPT = `[COMPACTION CONTEXT INJECTION]
+
+When summarizing this session, you MUST include the following sections in your summary:
+
+## 1. User Requests (As-Is)
+- List all original user requests exactly as they were stated
+- Preserve the user's exact wording and intent
+
+## 2. Final Goal
+- What the user ultimately wanted to achieve
+- The end result or deliverable expected
+
+## 3. Work Completed
+- What has been done so far
+- Files created/modified
+- Features implemented
+- Problems solved
+
+## 4. Remaining Tasks
+- What still needs to be done
+- Pending items from the original request
+- Follow-up tasks identified during the work
+
+## 5. MUST NOT Do (Critical Constraints)
+- Things that were explicitly forbidden
+- Approaches that failed and should not be retried
+- User's explicit restrictions or preferences
+- Anti-patterns identified during the session
+
+This context is critical for maintaining continuity after compaction.
+`
+
+export function createCompactionContextInjector() {
+  return async (ctx: SummarizeContext): Promise<void> => {
+    log("[compaction-context-injector] injecting context", { sessionID: ctx.sessionID })
+
+    const success = injectHookMessage(ctx.sessionID, SUMMARIZE_CONTEXT_PROMPT, {
+      agent: "general",
+      model: { providerID: ctx.providerID, modelID: ctx.modelID },
+      path: { cwd: ctx.directory },
+    })
+
+    if (success) {
+      log("[compaction-context-injector] context injected", { sessionID: ctx.sessionID })
+    } else {
+      log("[compaction-context-injector] injection failed", { sessionID: ctx.sessionID })
+    }
+  }
+}