docs: expand scope and add comprehensive documentation

Create README.md and enhance AGENTS.md to position this as an
extensible framework for any Opencode skills and agents, not just
PARA/task management. Includes installation, development workflow,
code style guidelines, and Nix flake integration patterns.

.beads/metadata.json

@@ -0,0 +1,4 @@
+{
+  "database": "beads.db",
+  "jsonl_export": "issues.jsonl"
+}