fetch: require a destination for automatic tag following - #7448

Fixes #7436

Changes

• Require a destination-bearing active fetch refspec before AUTO follows tags
• Ignore passive configured mappings when explicit source-only refspecs are supplied
• Preserve ALL, explicit tag refspecs, FETCH_HEAD updates and the configured tag policy
• Clarify AUTO documentation and extend the existing source-only FETCH_HEAD test

Validation

• Original issue reproducer creates no tags with both Git and libgit2 after the fix
• Regression fails before the fix with 7 unexpected tags and passes afterward
• Debug and Release/static builds pass with ENABLE_WERROR=ON, including strict C90 header checks
• ./libgit2_tests -sfetchhead::nonetwork::create_when_refpecs_given passes
• ./libgit2_tests -sfetch -sfetchhead -snetwork::fetchlocal -snetwork::remote::local -sremote::fetch passes all 87 cases
• Related fetch, network, remote and clone suites pass all 230 cases with both files and reftable backends
• ctest --output-on-failure -R '^(offline|util)$' passes in both builds
• Manual checks cover configured source-only refs, fallback HEAD, mixed refspecs, ALL and split download/update APIs
• Smart-transport checks match Git for source-only fetches, explicit tags and a reused remote

Existing Diagnostics

• Full-file Clang analysis reports the same 10 diagnostics as main, while analysis of the complete changed functions passes
• Destination-bearing local-transport fetches can still follow off-history tags, unchanged from main and not observed in the smart-transport checks