Feat/sqllite

[bharatsachya]

Pull Request Guide

## What changed

- Added `packages/sqlite` — a full SQLite implementation of `IDocumentDatabaseProvider` (knowledge, raw files, stats, activity, usage repositories + client + migrations)
- Introduced `packages/db-core` — shared TypeScript interfaces (`IDocumentDatabaseProvider` and its sub-repositories) that every document-DB provider must implement
- Introduced `packages/db` — a provider registry (`registerDbProvider` / `connectDb` / `getDb`) that decouples call-sites from any specific DB driver
- Introduced `packages/graph-core` — shared interfaces for the graph layer (`IGraphDatabaseProvider` and sub-repositories)
- Introduced `packages/graph-db` — a parallel provider registry for the graph layer (`registerGraphProvider` / `connectGraph` / `getGraph`)
- Refactored `packages/mongo` to implement `IDocumentDatabaseProvider` and self-register via `registerDbProvider("mongo", ...)`
- Refactored `packages/neo4j` to implement `IGraphDatabaseProvider` and self-register via `registerGraphProvider("neo4j", ...)`
- Updated `packages/server` and `packages/ingest-github` bootstrap to connect through the registry (`connectDb(dbProvider)` / `connectGraph(graphProvider)`) instead of hard-wiring Mongo/Neo4j imports
- Added three new config keys: `db_provider` (default `"sqlite"`), `graph_provider` (default `"neo4j"`), `sqlite_path`; `MONGO_URI` is no longer required when `db_provider !== "mongo"`
- Extracted all inline `readField` switch cases from `schema.ts` into a generated `schema-fields.ts` to reduce noise in the schema file
- Bumped `tsconfig.json` references across all affected packages

## Why

Mongo is a hosted dependency that adds friction for local development and self-hosted deployments. SQLite lets developers (and users running ByteBell on their own machines) spin up a fully functional instance with zero infrastructure. The provider registry pattern keeps every consumer (`server`, `queue`, `ingest-github`) DB-agnostic so future backends (Postgres, etc.) can be dropped in without touching call-sites.

## How to test

**SQLite path (new default)**

1. Make sure `db_provider` is unset or explicitly `sqlite` in your config:

bytebell set db_provider sqlite
bytebell set sqlite_path ~/.bytebell/data.sqlite # or leave blank for default

2. Start the server — no Mongo URI required:

bun run dev

3. Trigger a GitHub index job via `POST /github/index` and confirm it completes without errors
4. Hit `GET /health` — `db` ping should return `ok: true`
5. Open `sqlite3 ~/.bytebell/data.sqlite` and verify rows are present in the `knowledge`, `raw_files`, and `usage` tables

**Mongo path (regression)**

1. Set `db_provider mongo` and supply a valid `MONGO_URI`
2. Repeat steps 2–4 above — behaviour should be identical to pre-release

**Graph provider (Neo4j — no change expected)**

1. Confirm `graph_provider` defaults to `neo4j` and knowledge nodes still appear in Neo4j after an index run

**Config validation**

1. Start the server with `db_provider mongo` but no `MONGO_URI` — server should refuse to start with a missing-config error
2. Start with `db_provider sqlite` and no `MONGO_URI` — server should start cleanly

[bharatsachya]

Against #56