Add transient-failure retry to git clone/fetch in git-source.ts

[thepushkaraj]

Split out from #34. The retry core (#43) wraps the URL fetcher, embeddings, and Pinecone upsert, but the git source clone/fetch path is not yet retried.

packages/template/src/sources/git-source.ts runs git via synchronous execFileSync. A failed clone throws an exec error carrying a non-zero exit + stderr — not the ECONNRESET/ETIMEDOUT codes that withRetry's default isRetryableError recognizes. So this needs a git-specific retry predicate that only retries the network ops (clone/fetch), not the local reset/scrub steps, plus tests.

Now shared by both GitHub and GitLab fetchers (post-#46), so one change covers both.