feat: agent loop harness with Claude Code plugin support

Generator-evaluator architecture with iterative context-reset for
long-running coding tasks. Ships as a Claude Code plugin — install
with /plugin and use /agent-loop:init, /agent-loop:plan, /agent-loop:run.

.claude-plugin/marketplace.json

@@ -0,0 +1,14 @@
+{
+  "name": "agent-loop",
+  "plugins": [
+    {
+      "name": "agent-loop",
+      "description": "Autonomous generator-evaluator agent loop for long-running coding tasks. Plan interactively, then execute with full visibility.",
+      "version": "0.1.0",
+      "source": {
+        "source": "github",
+        "repo": "https://git.jagfly.com/sheldon/loop-loop.git"
+      }
+    }
+  ]
+}