hub: default auto-created repos to internal source

[Vishnu Suresh (vishnu-ssuresh)]

Summary

• default hub auto-created repos to source: "internal"
• switch hub repo creation path to raw POST /api/v1/repos so source can be set
• keep existing metadata behavior (description, readme, tags, is_public) and 409 conflict tolerance
• add test coverage asserting source is set to internal

Testing

• go test ./internal/cmd -run 'TestEnsureHubRepo|TestHubPush_CreatesRepoAndPostsCommit'

[Mateusz Kędzia (matercomus)]

This looks like the fix for #133 — confirming the bug from the user side.

langsmith hub push <new-name> --type agent creates a repo that langsmith hub list / langsmith hub get and the runtime SDK all see fine, but it never appears in the /context Context Hub UI ("No context yet"). Root cause matches this PR exactly: the CLI-pushed repo object has no source field, while UI-created repos have "source": "internal", and the /context list filters on it.

One observation worth preserving in the test coverage: pushing into an already-existing UI-created repo currently keeps its source: "internal" (the field is only set at creation, untouched by later commits). So when switching the creation path to raw POST /api/v1/repos, it'd be good to confirm the existing-repo update path still doesn't reset or drop source.