Implement OpenCode explicit skill capture

[Stephen Belanger (Qard)]

Summary

Captures explicit OpenCode skill invocations from the native /skills command and keeps the replay/event processor path in sync with the production tracing hooks.

• Records turn-level loaded_skill_names and loaded_skills for named commands such as /skills review.
• Treats bare /skills as an explicit invocation and backfills the selected skill name from the native skill tool call when it becomes available.
• Marks matching native skill tool spans with skill_load_trigger: "explicit".
• Preserves metadata.tool_name as the raw OpenCode tool name and adds metadata.tool_kind: "skill" for native skill-load spans.
• Preserves the natural-language behavior: prompts like Use the review skill remain implicit and do not set explicit skill metadata.

Validation

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