feat: auto-update harness files when plugin version changes

setup.sh now stamps .harness-version in .loop/ at scaffold time. On each
/agent-loop:run, Phase 1 compares the installed harness version against
the plugin version and auto-updates lib/, prompts/, and loop.sh if stale.
Run state (prd.json, contracts, config.json) is preserved.

Also adds setup.sh --update mode for refreshing harness files without
re-scaffolding. Bump to 0.10.0.

.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "agent-loop",
-  "version": "0.9.0",
+  "version": "0.10.0",
   "description": "Autonomous generator-evaluator agent loop for long-running coding tasks. Run /agent-loop:run to start.",
   "author": {
     "name": "Sheldon"