feat(rules): add rules directory structure and usage documentation

- Create rules/{concerns,languages,frameworks}/ directory structure
- Add USAGE.md with flake.nix integration examples
- Add plan and notepad files for rules-system implementation

Refs: T1, T5 of rules-system plan

rules/USAGE.md

@@ -0,0 +1,62 @@
+# Opencode Rules Usage
+
+Add AI coding rules to your project via `mkOpencodeRules`.
+
+## flake.nix Setup
+
+```nix
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+    m3ta-nixpkgs.url = "git+https://code.m3ta.dev/m3tam3re/nixpkgs";
+    agents = {
+      url = "git+https://code.m3ta.dev/m3tam3re/AGENTS";
+      flake = false;
+    };
+  };
+
+  outputs = { self, nixpkgs, m3ta-nixpkgs, agents, ... }:
+    let
+      system = "x86_64-linux";
+      pkgs = nixpkgs.legacyPackages.${system};
+      m3taLib = m3ta-nixpkgs.lib.${system};
+    in {
+      devShells.${system}.default = let
+        rules = m3taLib.opencode-rules.mkOpencodeRules {
+          inherit agents;
+          languages = [ "python" "typescript" ];
+          frameworks = [ "n8n" ];
+        };
+      in pkgs.mkShell {
+        shellHook = rules.shellHook;
+      };
+    };
+}
+```
+
+## Parameters
+
+- `agents` (required): Path to AGENTS repo flake input
+- `languages` (optional): List of language names (e.g., `["python" "typescript"]`)
+- `concerns` (optional): Rule categories (default: all standard concerns)
+- `frameworks` (optional): List of framework names (e.g., `["n8n" "django"]`)
+- `extraInstructions` (optional): Additional instruction file paths
+
+## .gitignore
+
+Add to your project's `.gitignore`:
+```
+.opencode-rules
+opencode.json
+```
+
+## Project Overrides
+
+Create `AGENTS.md` in your project root to override central rules. OpenCode applies project-level rules with precedence over central ones.
+
+## Updating Rules
+
+When central rules are updated:
+```bash
+nix flake update agents
+```