## Summary
- **Username-prefix branches**: Local visual-diff builds now use
`charles/main` instead of `main` as the branch name, preventing local
runs from creating auto-approved reference builds that could overwrite
CI baselines.
- **Local merge-base computation**: Computes `ARGOS_REFERENCE_COMMIT`
via `git merge-base HEAD main` locally, so the Argos SDK skips `git
fetch origin <branch>` — fixing the "fatal: couldn't find remote ref"
error when running from non-pushed branches.
- **Pass `referenceCommit` to vitest plugin**: Ensures the locally
computed merge-base is forwarded to the Argos upload.
## Test plan
- [x] Verified local visual-diff works from `main` branch (branch
becomes `charles/main`, not auto-approved)
- [x] Verified local visual-diff works from a non-pushed branch
(`test/local-only-visual-diff` → build uploaded successfully)
