docs(zi-install): document setup apply events interface

[ss-o]

Summary

Document the additive setup.sh apply --events DIR interface in .github/skills/zi-install/SKILL.md:

• Clarify that zi-setup-describe-v1 and zi-setup-result-v1 remain the baseline from z-shell/src#224, while zi-setup-event-v1 is explicitly an optional additive capability from z-shell/src#225.
• In the event bullet, instruct clients to treat process completion as authoritative and tolerate a missing terminal event when a cancellation signal interrupts event publication itself; staging cleanup and exit 6 still apply.

Refs: z-shell/src#225