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/test/agent-session-compaction.test.ts

@@ -156,9 +156,9 @@ describe.skipIf(!API_KEY)("AgentSession compaction e2e", () => {
 		expect(compaction.type).toBe("compaction");
 		if (compaction.type === "compaction") {
 			expect(compaction.summary.length).toBeGreaterThan(0);
-			// firstKeptEntryIndex can be 0 if all messages fit within keepRecentTokens
+			// firstKeptEntryId can be 0 if all messages fit within keepRecentTokens
 			// (which is the case for small conversations)
-			expect(compaction.firstKeptEntryIndex).toBeGreaterThanOrEqual(0);
+			expect(compaction.firstKeptEntryId).toBeGreaterThanOrEqual(0);
 			expect(compaction.tokensBefore).toBeGreaterThan(0);
 		}
 	}, 120000);