Create mainframe push reservation

Acquires a distributed reservation required for batched mainframe push requests. Validates that the client's snapshot baseline matches the remote state. If drift is detected (canReserve=false), returns conflict details without acquiring a lock. If baseline matches, reserves exclusive access and returns a reservationId valid for 5 minutes.

Features:

• Distributed reservation locking (Redis-backed TTL of 5 minutes)
• Preflight fingerprint validation against the current remote snapshot
• Snapshot drift detection: returns canReserve=false with remote snapshot details on mismatch
• Hidden/system files are excluded from fingerprint evaluation by default (includeHidden=false)
• Concurrent push blocking: 409 response if another push already holds the lock
• Atomic check-and-lock pattern: ensures the baseline matches before the reservation is acquired
• Heartbeat extension during multi-batch push (each batch extends TTL)

Use Cases:

• Preflight validation before batching multi-file push to remote
• Blocking concurrent push attempts to same pipeline
• Obtaining unique reservationId for multi-phase orchestration
• Detecting stale local snapshot before starting expensive push operations

Workflow:

1. Extension computes local snapshot fingerprint
2. POST /push-reservation with fingerprint
3. If canReserve=true: proceed with batched push, use reservationId for each batch
4. If canReserve=false: show snapshot drift with remote files, prompt user for refresh/rebase

Required Permissions: replication-pipeline::create when permission harmonization is enabled; otherwise, replication-pipeline::manage.