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/src/harness/session/memory-storage.ts

@@ -1,6 +1,7 @@
 import { uuidv7 } from "@earendil-works/pi-ai";
 import {
 	type LeafEntry,
+	type SessionEntryCursorOptions,
 	SessionError,
 	type SessionMetadata,
 	type SessionStorage,
@@ -112,13 +113,66 @@ export class InMemorySessionStorage<TMetadata extends SessionMetadata = SessionM
 		return this.labelsById.get(id);
 	}
 
-	async getPathToRoot(leafId: string | null): Promise<SessionTreeEntry[]> {
+	async getSessionName(): Promise<string | undefined> {
+		const entries = await this.findEntries("session_info");
+		return entries[entries.length - 1]?.name?.trim() || undefined;
+	}
+
+	async getSessionStats() {
+		let messageCount = 0;
+		let cachedTokens = 0;
+		let uncachedTokens = 0;
+		let totalTokens = 0;
+		let costTotal = 0;
+		for (const entry of this.entries) {
+			if (entry.type === "message") {
+				messageCount += 1;
+			}
+			const usage =
+				entry.type === "message"
+					? entry.message.role === "assistant"
+						? entry.message.usage
+						: undefined
+					: entry.type === "compaction" || entry.type === "branch_summary"
+						? entry.usage
+						: undefined;
+			if (
+				!usage ||
+				typeof usage.input !== "number" ||
+				typeof usage.output !== "number" ||
+				typeof usage.cacheRead !== "number" ||
+				typeof usage.cacheWrite !== "number" ||
+				typeof usage.cost?.total !== "number"
+			) {
+				continue;
+			}
+			cachedTokens += usage.cacheRead;
+			uncachedTokens += usage.input + usage.cacheWrite;
+			totalTokens += usage.input + usage.output + usage.cacheRead + usage.cacheWrite;
+			costTotal += usage.cost.total;
+		}
+		return {
+			messageCount,
+			cachedTokens,
+			uncachedTokens,
+			totalTokens,
+			costTotal,
+		};
+	}
+
+	async getPathToRootOrCompaction(leafId: string | null): Promise<SessionTreeEntry[]> {
 		if (leafId === null) return [];
 		const path: SessionTreeEntry[] = [];
+		let stopAtEntryId: string | null = null;
 		let current = this.byId.get(leafId);
 		if (!current) throw new SessionError("not_found", `Entry ${leafId} not found`);
 		while (current) {
 			path.unshift(current);
+			if (stopAtEntryId !== null && current.id === stopAtEntryId) break;
+			if (current.type === "compaction") {
+				if (current.retainedTail) break;
+				stopAtEntryId = current.firstKeptEntryId ?? null;
+			}
 			if (!current.parentId) break;
 			const parent = this.byId.get(current.parentId);
 			if (!parent) throw new SessionError("invalid_session", `Entry ${current.parentId} not found`);
@@ -127,7 +181,9 @@ export class InMemorySessionStorage<TMetadata extends SessionMetadata = SessionM
 		return path;
 	}
 
-	async getEntries(): Promise<SessionTreeEntry[]> {
-		return [...this.entries];
+	async getEntries(options?: SessionEntryCursorOptions): Promise<SessionTreeEntry[]> {
+		const start = options?.afterEntrySeq ?? 0;
+		const end = options?.limit === undefined ? undefined : start + options.limit;
+		return this.entries.slice(start, end);
 	}
 }