official Packages: make the example confirm activation at login #330

This was generated by AI during triage.

Agent Brief

Category: enhancement
Summary: Make the reference Package prove its login hook through a private client System Message.

Current behavior:
The Official Package Collection's example Package registers on_login and reads its typed payload. CI proves discovery and compilation, but a client cannot observe that the hook fired.

Desired behavior:
After core exposes the Package-callable System Message operation, update example so its login hook sends one short activation message to the character logging in. Use the character guid from the event payload. The message must be private and must not impersonate character chat.

Acceptance criteria:

• Logging in with example enabled shows one System Message stating that the example Package is active.
• Other connected characters receive nothing.
• The hook uses the core System Message operation rather than inserting event rows directly.
• The Package remains free of durable tables, reducers, client files, and Blizzard assets.
• Official Package core-tip CI passes.
• Disabling the Package removes the login message without core edits.

Out of scope:

• A configurable message
• Ordinary addon UI
• Say or whisper fallback behavior