Implement background service to auto-sync package operations between instances

[OceanOak]

This PR adds a background sync service that automatically synchronises package ops between Darklang instances. The sync service auto-starts when you launch either the CLI or the VS Code extension.

• It syncs all branches, not just the current one
• Batches Ops by (branchID, instanceID) to track origin and prevent duplicates
• Adds a CLI config system for sync settings

Test:

• Start CLI or open VS Code extension, verify sync service auto-starts (check sync-service.log)
• Push check : make changes in local instance, verify they appear on instance2 (check if http://dark-packages.dlio.localhost:11003/stats matches http://dark-packages.dlio.localhost:11001/stats)
• Pull check: make changes on instance2 (run DARK_CONFIG_DB_NAME=data-instance2.db ./scripts/run-cli val Stachu.Test.syncTestValue "\"Created in instance2\"", verify they appear on local instance (by checking logs or stats)
• Stop CLI or close VS Code, verify sync service stops gracefully

[StachuDotNet]

Generally, this is great. Just one notable gripe: as of this PR, the extension is the thing that's managing the sync service - and the sync service has bled a bit into the language server itself.

Rather,

• (long-term) the sync service should be managed by some broader 'dev server' that happens to serve the language server, mcp server(s), and sync. So as part of a CLI install, someone should be able to type dark dev-server start
• for now, let's just have a tiny separate CLI app that just does sync - takes a 'start' or 'stop' command/arg. Then, we can remove the LSP way of doing things, and have the extension call upon the separate CLI app rather than do things via a LSP-centric handshake
• I'm not sure what to do about the config file -- if syncing is 'separate' where would those things live? I'm open to any options, incl. push-back against it being a 'separate app' but would be happy to think through other options.
• the 'main' CLI can call upon the 'separate' syncing CLI app, or something?
• certainly, the main CLI app shouldn't have a sync-service-loop thing exposed in the commands (in core.dark)

We're in a sort of in-between awkward stage, and will end up iterating here a few times over the next weeks and beyond, so any progress in these directions is fine / good enough.

Tangentially - do we still need vscode-extension/client/src/commands/instanceCommands.ts? (Seems empty)