refactor: add write_job_sh helper to PEcAn.utils to reduce duplicated job.sh logic

[KrupaShah20]

Summary

Closes #3887

This PR adds a shared helper function write_job_sh() to PEcAn.utils
and refactors all 17 model couplers to use it instead of duplicated
job.sh writing logic.

Problem

Every write.config.* function duplicated these same lines:

• writeLines(jobsh, con = file.path(...))
• Sys.chmod(file.path(...))

This pattern was repeated across 17 different model packages making
the codebase harder to maintain and extend.

Solution

• Added base/utils/R/write_job_sh.R with a fully documented helper function
• Refactored all 17 model couplers to use PEcAn.utils::write_job_sh()
• Added unit tests in base/utils/tests/testthat/test.write_job_sh.R
• Maintained backward compatibility throughout

Models Refactored

template, sipnet, rothc, sibcasa, ed, ed2-restart, fates, gday,
jules, ldndc, linkages, lpjguess, maat, maespa, peprmt, preles, stics

Testing

Added unit tests covering:

• File creation verification
• File content correctness
• chmod parameter behavior