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/src/index.ts

@@ -0,0 +1,97 @@
+import type { SQLInputValue } from "node:sqlite";
+import { DatabaseSync } from "node:sqlite";
+import type { SqliteDatabase, SqliteDatabaseFactory, SqliteRunResult, SqliteStatement } from "./sqlite/types.ts";
+
+function isNamedParameters(value: unknown): value is Record<string, SQLInputValue> {
+	if (value === null || typeof value !== "object") return false;
+	if (Array.isArray(value) || ArrayBuffer.isView(value)) return false;
+	return true;
+}
+
+class NodeSqliteStatement implements SqliteStatement {
+	private readonly statement: ReturnType<DatabaseSync["prepare"]>;
+
+	constructor(statement: ReturnType<DatabaseSync["prepare"]>) {
+		this.statement = statement;
+	}
+
+	async run(...params: unknown[]): Promise<SqliteRunResult> {
+		const [first, ...rest] = params;
+		const result = isNamedParameters(first)
+			? this.statement.run(first, ...(rest as SQLInputValue[]))
+			: this.statement.run(...(params as SQLInputValue[]));
+		return {
+			changes: Number(result.changes),
+			lastInsertRowid: result.lastInsertRowid === undefined ? undefined : Number(result.lastInsertRowid),
+		};
+	}
+
+	async get<TRow extends object>(...params: unknown[]): Promise<TRow | undefined> {
+		const [first, ...rest] = params;
+		return (
+			isNamedParameters(first)
+				? this.statement.get(first, ...(rest as SQLInputValue[]))
+				: this.statement.get(...(params as SQLInputValue[]))
+		) as TRow | undefined;
+	}
+
+	async all<TRow extends object>(...params: unknown[]): Promise<TRow[]> {
+		const [first, ...rest] = params;
+		return (
+			isNamedParameters(first)
+				? this.statement.all(first, ...(rest as SQLInputValue[]))
+				: this.statement.all(...(params as SQLInputValue[]))
+		) as TRow[];
+	}
+}
+
+class NodeSqliteDatabase implements SqliteDatabase {
+	private readonly db: DatabaseSync;
+
+	constructor(db: DatabaseSync) {
+		this.db = db;
+	}
+
+	async exec(sql: string): Promise<void> {
+		this.db.exec(sql);
+	}
+
+	prepare(sql: string): SqliteStatement {
+		return new NodeSqliteStatement(this.db.prepare(sql));
+	}
+
+	async transaction<T>(fn: () => Promise<T>): Promise<T> {
+		this.db.exec("BEGIN");
+		try {
+			const result = await fn();
+			this.db.exec("COMMIT");
+			return result;
+		} catch (error) {
+			try {
+				this.db.exec("ROLLBACK");
+			} catch {
+				// Ignore rollback errors to rethrow original error.
+			}
+			throw error;
+		}
+	}
+
+	async close(): Promise<void> {
+		this.db.close();
+	}
+}
+
+export function wrapNodeSqliteDatabase(db: DatabaseSync): SqliteDatabase {
+	return new NodeSqliteDatabase(db);
+}
+
+export function createNodeSqliteFactory(): SqliteDatabaseFactory {
+	return {
+		async open(path: string): Promise<SqliteDatabase> {
+			return new NodeSqliteDatabase(new DatabaseSync(path));
+		},
+	};
+}
+
+// Re-export the SQLite session storage backend and types so this package is a complete node-sqlite backend.
+export * from "./sqlite/index.ts";