fix(updates): report a plugin build installed beside the running one, never "up to date" on a failed check

A `claude plugin update` run from a terminal (or with only the Claude desktop
app open) unpacks the new build and starts nothing: no SessionStart fires, so
the dashboard kept running the old cache dir across reboots, and with the
forge unreachable the sidebar read "Up to date" (seen on a machine stuck at
1.8.10).

- pluginStatus asks supersededBy for its own root: a recorded sibling install
  sets `installed_not_running: {root, version}` and `update_available`, with
  no network needed
- the sidebar reads "<version> installed, not running"; the update modal keys
  on it, so Update now is offered even with no release tag or sha
- plugin-update-helper hands over to the recorded install first, the newest
  sibling dir only as a fallback
- a plugin install whose release lookup failed reads "Could not check for
  updates" instead of "Up to date"
- docs/PLUGINS.md: the case and the manual hand-over command

scripts/plugin-update-helper.js

@@ -158,7 +158,10 @@ function main() {
     return;
   }
 
-  const newRoot = findNewRoot(oldRoot);
+  // Claude Code's own install record first: newest-mtime is only a guess, and a
+  // build installed from a terminal while this server kept running is exactly
+  // the one to hand over to, whether or not this chain pulled anything.
+  const newRoot = require("./plugin-bootstrap").supersededBy(oldRoot) || findNewRoot(oldRoot);
   if (!newRoot) {
     // Already newest: the marketplace had nothing to pull. Not a failure — the
     // old server is still running and untouched, so say so plainly.