P1: Add task/resource reservations for gather and haul assignments

[itsmiso-ai]

Prevent multiple workers from duplicating gather/haul work against the same remaining resource or build need.

Parent: #121

Audit evidence:

• Worker task assignment independently chooses nearest task.
• Build haul tasks do not account for resources already reserved/carried by another worker.

Acceptance criteria:

• Track reserved build deliveries per resource.
• Clamp delivery to remaining need and return/refund excess.
• Regression test covers two workers targeting one remaining resource/build need.

[itsmiso-ai]

Resolved by PR #150 which implements the task/resource reservation system.

PR: #150

The implementation covers all acceptance criteria from this issue:

• Tracks reserved build deliveries per resource
• Clamps delivery to remaining need and refunds excess
• Regression test covers two workers targeting one remaining resource/build need