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/package.json

@@ -21,12 +21,11 @@
 		"README.md"
 	],
 	"scripts": {
-		"clean": "shx rm -rf dist",
 		"build": "tsgo -p tsconfig.build.json",
 		"test": "vitest --run",
 		"test:harness": "vitest --run --config vitest.harness.config.ts",
 		"coverage:harness": "vitest --run --config vitest.harness.config.ts --coverage",
-		"prepublishOnly": "npm run clean && npm run build"
+		"prepublishOnly": "npm run build"
 	},
 	"dependencies": {
 		"@earendil-works/pi-ai": "^0.80.10",