Implement explicit skill capture

[Stephen Belanger (Qard)]

Summary

Implements explicit skill request capture for Codex tracing while preserving raw tool names for observed skill loads.

• Records explicit skill requests on turn metadata via loaded_skill_names and loaded_skills.
• Marks matching skill-load spans with skill_load_trigger: "explicit".
• Preserves metadata.tool_name as the raw Codex tool name, such as exec_command or skills.read, and adds metadata.tool_kind: "skill" for skill-load classification.

Validation

• bun run check
• bun run typecheck
• bun test
• bun run build