refactor: remove dead code, extract shared agent options, optimize flake

- Remove dead overlays/default.nix (flake defines overlays inline)
- Remove orphaned overlays/mods/{beads,n8n}.nix (never imported)
- Remove docs/packages/notesmd-cli.md (package doesn't exist)
- Extract externalSkills submodule to shared-options.nix (eliminates
  ~100 lines of duplication across opencode/claude-code/pi modules)
- Fix lib output: use nixpkgs.lib directly instead of instantiating
  a full nixpkgs just to get lib
- Add lib unit tests to flake checks
- Update stale comment in coding-rules.nix

docs/packages/notesmd-cli.md

@@ -1,117 +0,0 @@
-# notesmd-cli
-
-Obsidian CLI (Community) - Interact with Obsidian in the terminal.
-
-## Description
-
-notesmd-cli is a command-line interface for interacting with Obsidian, the popular knowledge management and note-taking application. It allows you to create, search, and manipulate notes directly from the terminal.
-
-## Features
-
-- 📝 **Note Creation**: Create new notes from the command line
-- 🔍 **Search**: Search through your Obsidian vault
-- 📂 **Vault Management**: Interact with your vault structure
-- 🔗 **WikiLink Support**: Work with Obsidian's WikiLink format
-- 🏷️ **Tag Support**: Manage and search by tags
-- ⚡ **Fast**: Lightweight Go binary with no external dependencies
-
-## Installation
-
-### Via Overlay
-
-```nix
-{pkgs, ...}: {
-  environment.systemPackages = with pkgs; [
-    notesmd-cli
-  ];
-}
-```
-
-### Direct Reference
-
-```nix
-{pkgs, ...}: {
-  environment.systemPackages = with pkgs; [
-    inputs.m3ta-nixpkgs.packages.${pkgs.system}.notesmd-cli
-  ];
-}
-```
-
-### Run Directly
-
-```bash
-nix run git+https://code.m3ta.dev/m3tam3re/nixpkgs#notesmd-cli
-```
-
-## Usage
-
-### Basic Commands
-
-```bash
-# Show help
-notesmd-cli --help
-
-# Create a new note
-notesmd-cli new "My Note Title"
-
-# Search notes
-notesmd-cli search "search term"
-
-# List notes
-notesmd-cli list
-```
-
-### Working with Vaults
-
-```bash
-# Specify vault path
-notesmd-cli --vault /path/to/vault new "Note Title"
-
-# Open a note in Obsidian
-notesmd-cli open "Note Name"
-```
-
-### Advanced Usage
-
-```bash
-# Search with tags
-notesmd-cli search --tag "project"
-
-# Append to existing note
-notesmd-cli append "Note Name" "Additional content"
-```
-
-## Configuration
-
-### Environment Variables
-
-- `OBSIDIAN_VAULT`: Default vault path
-
-### Command Line Options
-
-Run `notesmd-cli --help` for a complete list of options.
-
-## Build Information
-
-- **Version**: 0.3.0
-- **Language**: Go
-- **License**: MIT
-- **Source**: [GitHub](https://github.com/Yakitrak/notesmd-cli)
-- **Vendor Hash**: null (no external dependencies)
-
-## Platform Support
-
-- Linux
-- macOS (Unix systems)
-
-## Notes
-
-- No vendor dependencies (pure Go stdlib)
-- The binary is named `notesmd-cli` (not `notesmd`)
-- This is the community CLI, not the official Obsidian CLI
-
-## Related
-
-- [Obsidian](https://obsidian.md) - The Obsidian application
-- [Adding Packages](../guides/adding-packages.md) - How to add new packages
-- [Quick Start](../QUICKSTART.md) - Getting started guide