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/vitest.harness.config.ts

@@ -3,6 +3,7 @@ import { defineConfig } from "vitest/config";
 
 const aiSrcIndex = fileURLToPath(new URL("../ai/src/index.ts", import.meta.url));
 const aiSrcCompat = fileURLToPath(new URL("../ai/src/compat.ts", import.meta.url));
+const agentSrcIndex = fileURLToPath(new URL("../agent/src/index.ts", import.meta.url));
 
 export default defineConfig({
 	test: {
@@ -21,6 +22,7 @@ export default defineConfig({
 	},
 	resolve: {
 		alias: [
+			{ find: /^@earendil-works\/pi-agent-core$/, replacement: agentSrcIndex },
 			{ find: /^@earendil-works\/pi-ai$/, replacement: aiSrcIndex },
 			{ find: /^@earendil-works\/pi-ai\/compat$/, replacement: aiSrcCompat },
 		],