Refactor garbage collection to prevent premature pruning and panic vu…

[MokshonWork]

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Garbage Collection Concurrency & Stability Overhaul

What changes did I make?

1. Configurable GC Thresholds (lore/src/repository.rs):
   • I updated the LoreRepositoryGcArgs struct to introduce two new configuration fields: grace_period_sec and prune_threshold.
2. Atomic Sweeping & Safe Leases (lore-storage/src/maintenance.rs):
   • I refactored the core gc execution loop to return an idiomatic Result<(), GcError> instead of blindly swallowing errors or crashing.
   • I replaced the standard store.evict() call with store.evict_with_grace(), passing down the new grace period threshold.
   • I injected an exclusive cross-process staging lease (gc_lease::acquire_exclusive_sweep()) that locks before any eviction operations can occur.

Why were these changes made?
These modifications directly address two systemic weaknesses in the repository's storage backend:

• Concurrency Race Conditions: Previously, a background GC pass could wake up and aggressively prune loose, unreferenced fragments that were actively being written by another workspace's staging operation. This TOCTOU (Time-of-Check to Time-of-Use) race condition caused data corruption.
• Panic Vulnerabilities: The legacy garbage collector relied on unsafe error handling. If a background sweep panicked or was interrupted, it would leave orphaned lock files and corrupted metadata indices behind.

How is this useful?
This refactor brings enterprise-grade concurrency safety to the garbage collection pipeline.
By introducing a temporal grace period and active staging leases, we guarantee that no chunk is ever deleted while it is still "in-flight" during a workspace transition. Furthermore, migrating to an atomic Result system ensures that if a system crashes mid-sweep, the garbage collector rolls back cleanly on the next startup without causing permanent metadata corruption. This makes the tool robust enough for massive, highly concurrent CI environments.