Session tree structure with id/parentId linking

- Add TreeNode base type with id, parentId, timestamp
- Add *Content types for clean input/output separation
- Entry types are now TreeNode & *Content intersections
- SessionManager assigns id/parentId on save, tracks leafId
- Add migrateSessionEntries() for v1 to v2 conversion
- Migration runs on load, rewrites file
- buildSessionContext() uses tree traversal from leaf
- Compaction returns CompactionResult (content only)
- Hooks return compaction content, not full entries
- Add firstKeptEntryId to before_compact hook event
- Update mom package for tree fields
- Better error messages for compaction failures

packages/coding-agent/examples/hooks/custom-compaction.ts

@@ -94,14 +94,12 @@ Format the summary as structured markdown with clear sections.`,
 				return;
 			}
 
-			// Return a compaction entry that discards ALL messages
-			// firstKeptEntryIndex points past all current entries
+			// Return compaction content - SessionManager adds id/parentId
+			// Use firstKeptEntryId from event to keep recent messages
 			return {
-				compactionEntry: {
-					type: "compaction" as const,
-					timestamp: new Date().toISOString(),
+				compaction: {
 					summary,
-					firstKeptEntryIndex: entries.length,
+					firstKeptEntryId: event.firstKeptEntryId,
 					tokensBefore,
 				},
 			};