Add create-mattermost-plugin skill to plugin-development plugin

Adds a new skill that scaffolds Mattermost plugins by cloning the
starter template, customizing module paths and plugin metadata, and
initializing a git repository with an initial commit.

Author: fmartingr

plugins/plugin-development/skills/create-mattermost-plugin/SKILL.md

@@ -0,0 +1,48 @@
+---
+name: create-mattermost-plugin
+description: Create a new Mattermost plugin from the starter template in the current directory. Use when creating a new plugin from scratch, scaffolding a Mattermost plugin, or bootstrapping a plugin project.
+user-invocable: true
+allowed-tools: Read, Write, Edit, Bash, Glob, Grep, AskUserQuestion
+---
+
+# Create Mattermost Plugin
+
+Scaffold a new Mattermost plugin by cloning the [starter template](https://github.com/mattermost/mattermost-plugin-starter-template) into the current directory and customizing it.
+
+The **plugin name** is the current folder name (e.g. if pwd is `/home/user/mattermost-plugin-foo`, the plugin name is `mattermost-plugin-foo`).
+The **Go module path** is `github.com/mattermost/<plugin-name>`.
+
+## Step 1: Pre-flight
+
+Check if pwd has files beyond dotfiles. If non-empty, warn the user with `AskUserQuestion` and let them choose to proceed or abort.
+
+## Step 2: Clone template into pwd
+
+```bash
+TMPDIR=$(mktemp -d)
+git clone --depth 1 https://github.com/mattermost/mattermost-plugin-starter-template "$TMPDIR"
+rm -rf "$TMPDIR/.git"
+cp -a "$TMPDIR"/. ./
+rm -rf "$TMPDIR"
+```
+
+## Step 3: Customize (follow README "Getting Started" steps)
+
+Derive values:
+
+- `PLUGIN_NAME` = basename of pwd (e.g. `mattermost-plugin-foo`)
+- `MODULE_PATH` = `github.com/mattermost/$PLUGIN_NAME`
+- `PLUGIN_ID` = `com.mattermost.$PLUGIN_NAME` (dots replaced for the id portion after `com.mattermost.`)
+
+1. **Edit `plugin.json`**: set `id`, `name`, `description` (use the plugin name as a sensible default for name/description, the user can refine later). Update `homepage_url` and `support_url` to point to `https://$MODULE_PATH` and `https://$MODULE_PATH/issues`.
+
+2. **Replace the old module path everywhere**: Use `Grep` to find all files containing `github.com/mattermost/mattermost-plugin-starter-template`, then `Edit` each with `replace_all` to substitute the new `MODULE_PATH`.
+
+## Step 4: Git init & first commit
+
+1. Run `git init` if `.git` doesn't exist
+2. `git add -A && git commit -m "Initial plugin scaffold from mattermost-plugin-starter-template"`
+
+## Step 5: Summary
+
+Print what was created and remind the user to run `make` to build.