feat: auto-archive completed runs before starting new features

When /agent-loop:run detects a previous run with all stories passed (or the
feature branch deleted after merge), it archives the old artifacts and resets
.loop/ automatically — no more manual rm -rf .loop.

- Add archive_and_reset() for on-demand archiving from skills
- Add runs.log index tracking all archived runs
- Update /run and /stories skills to detect completed runs
- setup.sh archives instead of hard-failing when prd.json exists
- Bump version to 0.9.0

.claude-plugin/marketplace.json

@@ -10,7 +10,7 @@
       "name": "agent-loop",
       "source": "./",
       "description": "Autonomous generator-evaluator agent loop for long-running coding tasks. Plan interactively, then execute with full visibility.",
-      "version": "0.8.0",
+      "version": "0.9.0",
       "author": {
         "name": "Sheldon"
       },

.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "agent-loop",
-  "version": "0.8.0",
+  "version": "0.9.0",
   "description": "Autonomous generator-evaluator agent loop for long-running coding tasks. Run /agent-loop:run to start.",
   "author": {
     "name": "Sheldon"