fix(opencode): skip nested repositories in snapshots

[opencode-agent]

Summary

• exclude directory-shaped untracked entries from legacy snapshot staging
• continue capturing ordinary untracked files in the same snapshot
• add coverage for an unborn nested Git repository

Why

Git reports embedded repositories as directory entries such as tui/. Passing one with no checked-out commit to the snapshot repository's git add fails the batch and leaves the snapshot tree stale. Since ls-files --others already expands ordinary directories into files, omitting directory entries avoids the failure without losing regular file changes.

Testing

• bun test test/snapshot/snapshot.test.ts
• bun typecheck
• bun x prettier --check src/snapshot/index.ts test/snapshot/snapshot.test.ts

Requested by: @rekram1-node (Aiden via Slack)

[Enough1122]

AI code review — automated review for reference, author can ignore or act on any point.

Nice targeted fix. In packages/opencode/src/snapshot/index.ts (hunk @@ -257,7, changed line 260), filtering directory-shaped entries out of all before staging addresses a real failure mode: an unborn embedded repository surfaces as tui/ from ls-files --others, and a single failed git add aborts the entire batch, leaving the snapshot stale. Since the list is NUL-split (-z style output), path quoting isn't a concern, and a trailing slash cannot belong to a legitimate file, so the filter is safe when applied to both the tracked and untracked halves of the set.

Two suggestions:

1. Nested-repo content now disappears silently instead of failing loudly. Consider logging a one-line warning when entries are dropped (count plus names) so users can understand why nothing under tui/ ever appears in a snapshot.
2. In test/snapshot/snapshot.test.ts (new test, roughly lines 177–195), a sibling case with a committed nested repo would pin down intended behavior there — git add currently accepts it as a gitlink — protecting against regression if the staging strategy changes later.

Also worth a quick audit that no other call site parses status --porcelain/ls-files output into staging candidates with the same latent bug, so snapshot behavior stays consistent across entry points.