fix: skip shadow repos for default branches - #11

Summary

• fetch the repository default_branch before deciding whether branch search needs a shadow repo
• skip shadow repo creation when the requested branch is already the default branch
• make the existing source-guard tests read index.ts from the current checkout instead of an author-local absolute path
• add regression coverage for default-branch, non-default, unavailable-default, disabled-branch-search, and omitted-branch decisions

Why

Issue #7 reports tmp-ghrtool-* shadow repos being created even for default branches. The execute path previously treated any provided branch as requiring shadow mode, so explicit main/dev requests could create a persistent shadow repo even when GitHub already indexes that branch as the repository default.

This keeps the old behavior when the default branch cannot be determined, so non-default branch search remains available instead of being silently skipped.

Partially addresses #7: this fixes default-branch detection. It does not attempt to solve shadow cleanup/TTL in the same change.

Validation

• bun install
• bun test (26 pass)
• git diff --check