Notify users when a plugin update is available (lazy, daily-throttled)

[JohnWeidner]

Problem

claude plugin list shows the installed version but not whether a newer version exists in the marketplace. Users only discover updates by running claude plugin marketplace update and claude plugin update <name> manually, which means installs silently drift behind.

Proposed behavior

When a wingspan skill or slash command is invoked, perform a lazy, throttled version check:

1. On PreToolUse for wingspan-scoped invocations, read a cache file (e.g. ~/.claude/plugin-state/vgv-wingspan/last-check).
2. If the cached timestamp is older than 24h, fetch the current version from the marketplace manifest and compare against the installed version.
3. If a newer version exists, print a single non-blocking line like:

   vgv-wingspan 0.0.2 is available (you have 0.0.1). Run \claude plugin update vgv-wingspan` to upgrade.`

4. Update the cache timestamp regardless of outcome.

Why this shape

• Lazy (not SessionStart): zero cost for users who don't use wingspan that day; avoids slowing session startup.
• Daily throttle: one reminder per day per plugin, not per invocation — avoids spam.
• In-context: the notice arrives while the user is already engaging with wingspan, so it's relevant rather than random.
• Single line, non-blocking: doesn't interrupt the workflow.

Notes

• Skills are markdown-as-instructions, so the check has to live in a real hook (executable), not inside skill markdown.
• If multiple VGV plugins adopt this pattern, a shared helper in a future "wingspan core" would keep the implementation DRY and let users silence all VGV update nudges with a single setting.

Alternative considered

A SessionStart hook was considered but rejected — it would run for every session whether or not the user touches wingspan that day, and if every plugin did the same, startup would get noisy fast.