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.

.gitignore

@@ -0,0 +1,19 @@
+# Runtime artifacts (generated per-project, not part of the harness)
+prd.json
+progress.md
+progress-archive.md
+config.json
+init.sh
+contracts/
+triage/
+archive/
+.archive-staging/
+.last-branch
+.loop.lock
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Claude Code
+.claude/