add --hub-only-solver-logs option

[DLWoodruff]

Summary

• New CLI flag --hub-only-solver-logs that, when paired with --solver-log-dir, causes only the hub to write per-subproblem solver logs; spokes (xhat evaluators, bounders, etc.) do not.
• Config.checker() rejects --hub-only-solver-logs if --solver-log-dir is not set.
• Implemented by giving cfg_vanilla.shared_options(cfg, is_hub=False) an is_hub parameter; ph_hub, subgradient_hub, and fwph_hub pass is_hub=True. When the flag is on and is_hub=False, solver_log_dir is dropped from the spoke options dict so spopt._solve_one skips log emission.

Motivation: a typical hub-and-spoke run with xhat spokes solves many small evaluation subproblems per outer iteration. With --solver-log-dir on, the spoke chatter dominates the log directory; this flag lets you keep hub subproblem logs without the spoke noise.

Test plan

• python -m pytest mpisppy/tests/test_config.py — 78 passed (6 new)
• ruff check . clean on changed files
• Checker rejects --hub-only-solver-logs alone
• Checker accepts the pair
• shared_options(cfg, is_hub=True) keeps solver_log_dir; is_hub=False drops it when hub_only_solver_logs is True

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 71.04%. Comparing base (bfc2ce3) to head (f840a43).

Files with missing lines	Patch %	Lines
mpisppy/utils/cfg_vanilla.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #682      +/-   ##
==========================================
+ Coverage   71.02%   71.04%   +0.02%     
==========================================
  Files         154      154              
  Lines       19248    19252       +4     
==========================================
+ Hits        13670    13677       +7     
+ Misses       5578     5575       -3     

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