[codex] Emit implicit skill usage for support reads

[alexsong-oai]

Summary

• Index all enabled skills for command-based usage detection, regardless of allow_implicit_invocation.
• Preserve allow_implicit_invocation for the model-visible implicit routing list.
• Add regression coverage for a support/preflight skill whose SKILL.md is read and whose script is run while implicit invocation is disabled.

Root cause

allow_implicit_invocation was used for both model routing and command-based usage-event detection. That meant support skills like data-analytics:user-context could be read or run by other skills, but those accesses could not emit implicit usage events.

Validation

• just fmt
• just test -p codex-core-skills service::tests::skills_for_config_indexes_usage_detection_for_non_implicit_skills
• just test -p codex-core-skills now has the new test passing, but 3 unrelated local tests fail because /Users/alexsong/.agents/skills/test/SKILL.md is invalid/missing YAML frontmatter.