git: make bad-tag recovery test deterministic

[OllieinCanada]

Fixes #350

Summary

Make the bad-tag recovery regression test deterministic across Git versions.

Root cause

The previous test depended on git fetch --prune --no-tags failing after a
corrupt local tag was created. Git 2.25.1 completes that fetch successfully
without inspecting or removing the tag, so Sashiko's recovery branch was never
entered and the assertion failed.

Changes

• extract the existing bad-tag deletion step into a private helper
• keep the production fetch and retry behavior unchanged
• invoke the recovery helper with the precise Git diagnostic in the test
• remove the unnecessary remote repository and version-dependent fetch setup

Validation

• reproduced the original failure with Git 2.25.1
• focused bad-tag recovery test passes
• all six git_ops tests pass
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features --release -- -D warnings
• cargo test --release
• git diff --check
• make check-pr