[FEATURE] per-plugin `lastUpdated` field in marketplace.json

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Summary

Plugin marketplaces that aggregate multiple plugins behind one marketplace.json cannot expose accurate per-plugin "last updated" dates in the Claude Code UI. When one plugin updates, every plugin in that marketplace appears to update at the same time — because Claude infers the date from marketplace-level metadata (the marketplace repo's HEAD commit time or marketplace.json mtime), not from per-plugin data.

Repro

1. Build a marketplace.json listing 5+ plugins behind a marketplace repo that receives plugin updates via repository_dispatch from per-plugin source repos.
2. Update only 1 plugin (bump its version + replace its plugin directory via dispatch, single PR merged to the marketplace repo).
3. Open the marketplace in Claude Code's /plugin UI.

Observed: all 5 plugins show today as the "last updated" date.
Expected: only the plugin that actually changed shows today; the others retain their previous dates.

Proposed Solution

Add an optional lastUpdated field (ISO 8601 timestamp) to the plugin entry schema in marketplace.json:

{
  "plugins": [
    {
      "name": "pax8-brand-voice",
      "source": "./plugins/pax8-brand-voice",
      "version": "1.2.3",
      "lastUpdated": "2026-05-11T19:30:00Z"
    }
  ]
}

Maintainers set this in their dispatch/publish automation. Claude Code's UI displays it when present; falls back to the current inferred date otherwise. Fully backwards-compatible.

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

Configuration and settings

Use Case Example

Users browsing the marketplace can't tell which plugin actually got the update. The signal that "this plugin was just refreshed" is lost, and stale plugins look freshly maintained — eroding trust in the marketplace's "last updated" column as a real signal.

Additional Context

_No response_