Track implicit skill loads

[Stephen Belanger (Qard)]

Summary

• detect observable Codex skill loads from skills.read, SKILL.md reads, and skill script executions
• normalize detected loads as observed skill-load tool spans while preserving the original tool name
• attach singular skill metadata including name/path when available
• omit the optional skill_load_trigger field for phase 1, so these loads default to implicit under the spec

Spec

• Document skill load metadata braintrust-spec#35

Validation

• bun test src/agents/codex/event-processor.test.ts
• bun run check