Add cache policy keep days to project creation

[jacobwgillespie]

Summary

• add --cache-policy-keep-days to depot projects create, matching the API's CachePolicy.KeepDays field
• send the configured retention in CachePolicy.KeepDays
• default retention to 14 days, reject negative values, and allow 0 for no limit

Resolves DEP-5626.

Testing

• go test ./pkg/cmd/projects
• go test ./...
• go fmt ./...
• go mod tidy
• go vet ./pkg/cmd/projects
• go run ./cmd/depot projects create --help

Linear Issue: DEP-5626

  

[linear-code]

DEP-5626

[cursor]

Approved. Cursor Bugbot and Cursor Security Agent completed successfully with no findings requiring human review. No reviewers were assigned.

  

_{Sent by Cursor Approval Agent: Pull Request Router and Approver}