feat(ext/sw): universal commitSigned — isomorphic-git receive-pack push + SSHSIG

Add CommitFile/CommitSigner types + commitSigned to GitHost interface.
Implement git-push.ts (clone→add→commit({onSign=SSHSIG})→push over http/web)
and wire GiteaHost.commitSigned / GitHubHost.commitSigned. Tests: 7/7 incl.
golden-vector tie against the sshsig.test.ts byte-exact vector.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LKBbrAjmpVXMEK15pNi3Ha

extension/package.json

@@ -13,6 +13,7 @@
     "test:watch": "vitest"
   },
   "dependencies": {
+    "isomorphic-git": "^1.38.5",
     "jsqr": "^1.4.0"
   },
   "devDependencies": {