Allow grouped skill prefix commands

[cbusillo]

Summary

• allow command_prefix_contains to match multiple expected skill files in a single launch command
• keep rejecting unrelated work commands before the expected prefix is satisfied

Validation

• focused grouped-prefix assertion snippet
• python3 -m py_compile tools/code-exec-harness/harness.py
• jq empty tools/code-exec-harness/scenarios/skill-trigger-binding-smoke.json tools/code-exec-harness/scenarios/github-plan-smoke.json
• python3 tools/code-exec-harness/harness.py tools/code-exec-harness/scenarios/skill-trigger-binding-smoke.json --skill-root /Users/cbusillo/Developer/codex-skills --inherit-auth
• python3 tools/code-exec-harness/harness.py tools/code-exec-harness/scenarios/github-plan-smoke.json --skill-root /Users/cbusillo/Developer/codex-skills --inherit-auth
• ./build-fast.sh