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.

templates/progress.md.template

@@ -0,0 +1,13 @@
+# Progress
+
+## Codebase Patterns
+
+<!-- Consolidate reusable patterns discovered across iterations here.
+     This section is READ FIRST by every new iteration.
+     Keep it concise — patterns only, not implementation details. -->
+
+---
+
+## Session Log
+
+<!-- Each iteration appends a progress entry below. Never delete entries. -->