chore: v2.8.7 — Git SSH keys in tabs, safer Artifacts preview and search

- Settings → Git → Your git identity: SSH keys are one tab per host kind
  (GitHub · GitLab, AWS CodeCommit, More keys, Other hosts); opens on the
  AWS tab when only an RSA key exists.
- Artifacts: a file type the sandboxed preview cannot show (Office, zip,
  PDF) offers Download only, never a blank Preview.
- Artifacts search: `%`/`_`/`\` in a query match literally (LIKE ... ESCAPE).
- Skill frontmatter rewrite replaces a key with its continuation lines and
  quotes values YAML could misread.
- Plugin marketplace.json is read with a 1 MB streaming cap.
- Artifact lists look each owner up once.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "agenthub",
-  "version": "2.8.6",
+  "version": "2.8.7",
   "bin": {
     "agenthub": "bin/agenthub.js"
   },