fix(proving-ground): use local runx evolve skill

[auscaster]

Summary

• invoke the checked-out runx evolve skill by local path in proving-ground
• assemble optional answers arguments without tripping set -u when no answer file is present

Validation

• RUNX_ROOT=/Users/kam/dev/runx-oss-thread-story ASTER_ROOT=/Users/kam/dev/runx/aster ARTIFACT_DIR=/tmp/aster-proving-ground-test PROVING_GROUND_PROFILE=minimal bash scripts/proving-ground.sh
  • This runs with no $RUNX_ANSWERS_DIR, so it directly exercises the no-answer-file branch that previously tripped set -u.
• node scripts/summarize-proving-ground.mjs /tmp/aster-proving-ground-test
• npm run check
• git diff --check
• bash -n scripts/proving-ground.sh

[auscaster]

I'd like one more validation detail in the PR body: does the listed proving-ground run actually exercise the no-answer-file path? Since half the change is about avoiding set -u there, please either call out that PROVING_GROUND_PROFILE=minimal hits that branch or add a small targeted check for it.

Head SHA: e070994