Add targeted usage limit commands

[m-abdelwahab]

Summary

• Add railway usage limit status --target <agent|workspace>, with no target showing both usage limit types
• Require --target for railway usage limit set
• Wire agent usage limit status/set through the public agentUsage query and agentUsageLimitSet mutation
• Add target-specific validation and formatting for agent vs workspace limits
• Support cent-level dollar inputs for agent limits while preserving whole-dollar workspace limits

Notes

• Agent hard limit is required when setting agent limits
• Agent --hard 0 is allowed and blocks agent usage
• Workspace limits preserve existing behavior, including whole-dollar validation

Test plan

• cargo test usage
• cargo test
• Manually tested status/set variants against a logged-in workspace