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

.sisyphus/notepads/rules-system/learnings.md

@@ -0,0 +1,60 @@
+# Rules System - Learnings
+
+## 2026-02-17T17:50 Session Start
+
+### Architecture Pattern
+- Nix helper lives in nixpkgs repo (not AGENTS) - follows ports.nix pattern
+- AGENTS repo stays pure content (markdown rule files only)
+- Pattern: `{lib}: { mkOpencodeRules = ...; }`
+
+### Key Files
+- nixpkgs: `/home/m3tam3re/p/NIX/nixpkgs/lib/ports.nix` (reference pattern)
+- nixos-config: `/home/m3tam3re/p/NIX/nixos-config/home/features/coding/opencode.nix` (deployment)
+- AGENTS: `rules/` directory (content)
+
+### mkOpencodeRules Signature
+```nix
+mkOpencodeRules {
+  agents = inputs.agents;  # Non-flake input path
+  languages = [ "python" "typescript" ];
+  concerns ? [ "coding-style" "naming" "documentation" "testing" "git-workflow" "project-structure" ];
+  frameworks ? [ "n8n" ];
+  extraInstructions ? [];
+}
+```
+
+### Consumption Pattern
+```nix
+let
+  m3taLib = inputs.m3ta-nixpkgs.lib.${system};
+  rules = m3taLib.opencode-rules.mkOpencodeRules {
+    agents = inputs.agents;
+    languages = [ "python" ];
+  };
+in pkgs.mkShell { shellHook = rules.shellHook; }
+```
+
+### Wave 1: Directory Structure (2026-02-17T18:54)
+- Successfully created rules/ directory with subdirectories: concerns/, languages/, frameworks/
+- Added .gitkeep files to each subdirectory (git needs at least one file to track empty directories)
+- Pattern reference: followed skills/ directory structure convention
+- USAGE.md already existed in rules/ (created by previous wave)
+- AGENTS repo stays pure content - no Nix files added (as planned)
+- Verification: ls confirms all three .gitkeep files exist in proper locations
+
+### Wave 2: Nix Helper Implementation (2026-02-17T19:02)
+- Successfully created `/home/m3tam3re/p/NIX/nixpkgs/lib/opencode-rules.nix`
+- Followed ports.nix pattern EXACTLY: `{lib}: { mkOpencodeRules = ...; }`
+- Function signature: `{ agents, languages ? [], concerns ? [...], frameworks ? [], extraInstructions ? [] }`
+- Returns: `{ shellHook, instructions }`
+- Instructions list built using map functions for each category (concerns, languages, frameworks, extra)
+- ShellHook creates symlink `.opencode-rules` → `${agents}/rules` and generates `opencode.json` with `$schema`
+- JSON generation uses `builtins.toJSON opencodeConfig` where opencodeConfig = `{ "$schema" = "..."; inherit instructions; }`
+- Comprehensive doc comments added matching ports.nix style (multi-line comments with usage examples)
+- All paths relative to project root via `.opencode-rules/` prefix
+- Verification passed:
+  - `nix eval --impure` shows file loads and exposes `mkOpencodeRules`
+  - Function returns `{ instructions, shellHook }`
+  - Instructions list builds correctly (concerns + languages + frameworks + extra)
+  - `nix-instantiate --parse` validates syntax is correct
+- ShellHook contains both symlink creation and JSON generation (heredoc pattern)