fix: remove outer transaction wrapper in sync to prevent PGlite deadlock

[Jpumpkin1223]

Problem

When syncing more than 10 changed files, sync.ts wraps all importFile() calls inside a single engine.transaction():

const useTransaction = (filtered.added.length + filtered.modified.length) > 10;
if (useTransaction) {
  await engine.transaction(async () => { await processAddsModifies(); });
} else {
  await processAddsModifies();
}

However, importFile() opens its own inner transaction on the same DB connection. PGlite does not support nested transactions — the event loop hangs indefinitely with no error, no output, and no timeout.

This makes gbrain sync silently hang forever whenever a repo has more than 10 changed files.

Fix

Remove the conditional outer engine.transaction() wrapper. Each importFile() call is already individually atomic, so no outer wrapper is needed.

Reproduction

1. Have a git repo with 11+ changed markdown files tracked in gbrain
2. Run gbrain sync --repo <path>
3. Observe: process hangs at 0% CPU indefinitely

Testing

Verified locally with a repo containing 17 changed files — sync now completes successfully.