feat: append workspace critical rules to compaction summary

- Add readWorkspaceContextForSummary() to extract Session Startup + Red Lines from AGENTS.md
- Inject workspace context into compaction summary (limited to 2000 chars)
- Export extractSections() from post-compaction-context.ts for reuse
- Ensures compaction summary includes core rules needed for recovery

Part 1 of post-compaction context injection feature.

src/auto-reply/reply/post-compaction-context.ts

@@ -49,7 +49,7 @@ export async function readPostCompactionContext(workspaceDir: string): Promise<s
  * Skips content inside fenced code blocks.
  * Captures until the next heading of same or higher level, or end of string.
  */
-function extractSections(content: string, sectionNames: string[]): string[] {
+export function extractSections(content: string, sectionNames: string[]): string[] {
   const results: string[] = [];
   const lines = content.split("\n");