feat: sqlite session storage (#6594)

This PR:

- Adds retainedTail to compaction entries in the new agent harness so we don't have to walk up the tree for the 2000 tokens before compaction,
- Changes getPathToRoot to getPathToRootOrCompaction to only load until last compaction, as unnecessary to access all nodes where it is called,
- Adds a SQLite storage backend, in a separate packages/session-backend-sqlite, with a migration system and schemas as per on-site discussions: sessions to match session header messages (except for metadata, which I couldn't understand what it's used for or where it gets written, so I omitted it), session_entries for shared entry types as columns plus payload as a json for what remains, session_sequences to represent the append-only, serialized nature of the jsonl files, branch_entries to attribute nodes to branches (relationship one-to-many), and session_materialized with the session info (see /session in TUI) to act as a "cache" or quick-access for costs, message count, token info, labels, session name, and model-thinking-level config (e.g. for fast resume).
- This is compatible with the new agent harness Session abstraction.

packages/agent/test/harness/session.test.ts

@@ -68,11 +68,20 @@ async function runSessionSuite(
 			await session.appendMessage(createAssistantMessage("two"));
 			const user2 = await session.appendMessage(createUserMessage("three"));
 			await session.appendMessage(createAssistantMessage("four"));
-			await session.appendCompaction("summary", user2, 1234);
+			await session.appendCompaction("summary", user2, 1234, undefined, undefined, undefined, [
+				createUserMessage("three"),
+				createAssistantMessage("four"),
+			]);
 			await session.appendMessage(createUserMessage("five"));
 			const context = await session.buildContext();
 			expect(context.messages[0]?.role).toBe("compactionSummary");
 			expect(context.messages).toHaveLength(4);
+			expect(context.messages.map((message) => message.role)).toEqual([
+				"compactionSummary",
+				"user",
+				"assistant",
+				"user",
+			]);
 		});
 
 		it("supports moving with branch summary entries in context", async () => {