Add --deps-only flag to separate dependency fetching from source builds

[sbryngelson]

Summary

• Adds --deps-only CLI flag to ./mfc.sh build that only fetches and builds dependencies (FFTW, HDF5, etc.) without building MFC source targets
• Already-configured dependencies are skipped during regular builds, preventing network access in the source build step
• Frontier and Frontier AMD CI now fetch deps on login nodes (internet access) and build source on compute nodes (no network needed)

Test plan

• Verify ./mfc.sh build --deps-only -j 8 builds only dependency targets
• Verify regular ./mfc.sh build -j 8 skips already-configured deps
• Verify Frontier CI: deps fetched on login node, source built + tested on compute node
• Verify Frontier AMD CI: same pattern via symlinked build.sh
• Verify Phoenix CI is unaffected (no login-node build step)
• Verify bench.yml Frontier/Frontier AMD jobs work with updated build_script args

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

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Run ID: b82e2aac-df52-4afb-83cf-cd069bb747b9

📥 Commits

Reviewing files that changed from the base of the PR and between 90d4b6e and a22f206.

📒 Files selected for processing (7)

• .github/workflows/bench.yml
• .github/workflows/common/bench.sh
• .github/workflows/common/test.sh
• .github/workflows/frontier/build.sh
• .github/workflows/test.yml
• toolchain/mfc/build.py
• toolchain/mfc/cli/commands.py

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📝 Walkthrough

Walkthrough

This pull request refactors the CI/CD build workflows to separate dependency fetching from source building. Changes include removing the bench argument from Frontier build commands in the benchmarking workflow, updating workflow step labels from "Setup & Build" to "Fetch Dependencies" and "Test" to "Build & Test", removing conditional build-directory checks to enable unconditional builds, simplifying the Frontier build script to only fetch dependencies without the run_bench parameter, and adding a new --deps-only flag to the Python build CLI command that resolves and builds only dependencies without building MFC targets. The toolchain's build module now includes early-exit logic for already-configured dependencies and a dedicated control flow for the deps_only mode.

🚥 Pre-merge checks | ✅ 1 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The PR description includes a clear summary, test plan with verification items, and addresses the motivation. However, it lacks the required template sections like 'Type of change', proper 'Testing' section structure, and the checklist items are incomplete.	Complete the description template by adding 'Type of change' section (mark New feature), expand 'Testing' with actual test results, and check off the verification items or provide test evidence.
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (1 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: adding a --deps-only flag to separate dependency fetching from source builds, which is the core feature across all modified files.

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