Add purgatory sync test helpers

Add tests/common/purgatory_helpers.rs with utilities for purgatory sync
integration tests:

- Git repository setup helpers (create_test_repo_with_commit,
  add_commit_to_repo, create_branch) for deterministic test commits
- State event creation (create_state_event) for kind 30618 events with
  refs/heads/*, refs/tags/*, and HEAD tags
- PR event creation (create_pr_event) for kind 1618 events with a and c tags
- Purgatory state inspection helpers (wait_for_event_served,
  verify_event_not_served) for polling event availability
- Git ref verification (check_ref_at_commit) for validating remote refs
- Push helper (push_to_relay) for pushing local repos to relay

All helpers include comprehensive unit tests verifying correct tag
structure and git operations.

1 files changed, 2 insertions, 0 deletions

diff --git a/tests/common/mod.rs b/tests/common/mod.rs
index 37ac3bb..f511163 100644
--- a/tests/common/mod.rs
+++ b/tests/common/mod.rs
@@ -2,8 +2,10 @@
 #![allow(dead_code)] // Test helpers may not be used in all test configurations
 #![allow(unused_imports)] // Re-exports may not be used in all test configurations
 
+pub mod purgatory_helpers;
 pub mod relay;
 pub mod sync_helpers;
 
+pub use purgatory_helpers::*;
 pub use relay::TestRelay;
 pub use sync_helpers::*;