test: add live integration tests for reachability checks

Uses nodeselector/actions-test-fixtures with:
- tag v1 on main HEAD (identical → reachable)
- root commit of main (ancestor → reachable)
- orphan-poison branch commit (no common ancestor → unreachable)
- fabricated SHA (404 → unreachable)
- cache consistency verification

Guarded by //go:build integration — won't run without -tags integration.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: nodeselector