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/storage/sqlite-node/scripts/prepare-dist.mjs

@@ -0,0 +1,35 @@
+#!/usr/bin/env node
+
+import { cp, mkdir, rm } from "node:fs/promises";
+import { dirname, resolve } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const scriptDir = dirname(fileURLToPath(import.meta.url));
+const packageDir = resolve(scriptDir, "..");
+const distDir = resolve(packageDir, "dist");
+const migrationSourceDir = resolve(packageDir, "src/sqlite/migrations");
+const migrationDestDir = resolve(distDir, "sqlite/migrations");
+
+async function clean() {
+	await rm(distDir, { recursive: true, force: true });
+}
+
+async function copySqliteMigrations() {
+	await mkdir(migrationDestDir, { recursive: true });
+	await cp(migrationSourceDir, migrationDestDir, { recursive: true });
+}
+
+const command = process.argv[2];
+
+if (command === "clean") {
+	await clean();
+	process.exit(0);
+}
+
+if (command === "copy-sqlite-migrations") {
+	await copySqliteMigrations();
+	process.exit(0);
+}
+
+console.error("Usage: node scripts/prepare-dist.mjs <clean|copy-sqlite-migrations>");
+process.exit(1);