Marketplace: derive the plugin slug from the id only (final review M4)

The slug (plugin marketplace "triple-c-<slug>", plugin tree
"plugins/<slug>/") was built from the editable display name. After a
rename the next sync registered the new marketplace, skipped the plugin
install because the state's commit matched, then removed the old
marketplace: the plugin was gone while the report said nothing changed.

marketplace_slug now takes the id only ("mp-<id8>"). With plugin state
kept per slug (I1), containers synced with the old "<name>-<id8>" slugs
move over on their next sync: plugins are installed under the new name,
the old copies uninstalled and the old registration dropped. A sync
script test covers that migration (it fails on the pre-I1 script).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/superpowers/specs/2026-09-27-marketplace-design.md

@@ -120,8 +120,8 @@ pub struct MarketplaceAccount {
     pub username: Option<String>, // resolved at sign-in, display only
 }
 pub struct Marketplace {
-    pub id: String,            // uuid
-    pub name: String,          // display; slug used in container paths
+    pub id: String,            // uuid; slug "mp-<id8>" used in container paths
+    pub name: String,          // display only (renaming never changes the slug)
     pub url: String,           // https URL only
     pub branch: Option<String>,// None = remote default branch
     pub account_id: Option<String>, // None = anonymous