Keep retention progressing when the oldest expired runs are held #500

A retention pass always selects the oldest expired, unarchived closed runs. A run that is skipped stays at the front of that order, and the next pass selects it again. When skipped runs fill a pass's limit, retention makes no progress on other eligible runs in that namespace.

Current behavior

Main 4e0f6458d890f4b34f1959c12dd385b2b1ea01f7:

• HistoryRetentionEnforcer::expiredRunIds orders candidates by closed_at with a limit and no exclusion for runs that were recently skipped or failed.
• The inline pass (runInlinePass) uses a limit of 1. One skipped run at the head stops inline pruning for that namespace.
• pruneRun can skip for reasons that persist until an operator acts, for example external_payload_storage_driver_unavailable or activity_policy_backend_unsupported.
• The scheduled history:prune pass uses a limit of 100, so it stalls only when at least that many such runs sit at the head. It does not print skip reasons and exits 0 when it only skipped.

This was found by reading source. It was not reproduced.

Expected

A held or failing run does not prevent pruning of other eligible runs. Operators can see which runs are held and why.

Acceptance

• With a persistently skipped run at the head, inline and scheduled passes still prune eligible runs behind it.
• Skipped and failed runs are reported with run id and reason through the retention status API and history:prune output. The command exits non-zero, or clearly reports no progress, when it made none.
• Skipped runs are retried later without a tight loop on every pass.
• Forever-retention, archived and hold semantics are unchanged.
• Coordinate Rust parity through Replace PHP Server with a compatible Rust runtime and safe in-place upgrade #325.